Welcome to My Fico forums, @cws-21!
You're asking for a rare bird with those three criteria. Mastercards in general seem to be a lot harder to find, and then those with great rewards and no FTF's are even more limiting! You also don't specify what type of rewards would best supplement your other cards. Here's my feedback, though, from what I know and on the above options.
It sounds like this is more of a card for traveling as a backup? If so, it sounds like the rewards aspect is secondary unless you find something very appealing. And in that case, it probably doesn't matter if the base level rewards are 1% or 1.5% or whatever, since it's not going to get a high enough level of spend to make a significant difference. Remember that as we go down the list.
Synchrony's PayPal Mastercard is a solid choice, although as you say it doesn't offer any higher level of rewards than your Citi Double Cash for daily spend.
From my understanding, there's no guarantee that many Capital One cards will be issued as either a Mastercard or a Visa. They've swung both ways. Also it sounds like the Capital One Savor One is a marginal addition to your lineup. And they do the dreaded triple-hard-pull of reports, if that is a consideration.
If you have an iPhone, the Apple card is a solid card for such a purpose. The physical card itself is very secure with no visible numbers. It's all managed through the phone. It's a Mastercard with no FTFs or other fees. Lower interest rates. Pays 2% to 3% when using Apple pay but always at least 1% with the physical card. A very compelling option.
ABOC Mastercard is a good suggestion, but usefulness depends on how the firm rotating categories would fit into your lineup. And of course, they are prone to change annually. The basic 1% back on rotating category cards is not competitive and you might be better off with some other options in my response if the 5% categories aren't highly useful to you.
Elan Max Cash Preferred can be a great card since you get to choose your categories for 5% cash back. The biggest downside in your case is probably 95% of the cards issued are VISA. However, the cards haven't shown to be geofenced by the smaller banks and credit unions offering it, so you can live in Maine and get a card from a California bank. The link to the thread is below and there are links within it to at least 75 lenders who offer the card. The one I know for sure that offers the card as a Mastercard is Bank of California. (See link below.) However, this version still DOES have a FTF of 2% of each foreign purchase transaction or foreign ATM advance transaction in U.S. Dollars; or 3% of each foreign purchase transaction or foreign ATM advance transaction in a Foreign Currency.)
https://bancofcal.com/banking/credit-cards/personal-credit-cards/#Max-Cash-Preferred
https://ficoforums.myfico.com/t5/Credit-Cards/Elan-quot-Max-Cash-quot-vs-US-Bank-Cash/td-p/6198795
Citibank is one of the largest issuers of Mastercards, but many of their cards do have the FTFs like your DoubleCash. Citi's Mastercards without FTFs are also travel cards with annual fees like the Premier and Prestige. This brings me back to the seeming contradiction of what you seek, since you're much more likely to find premium travel cards waive the FTF since they are for, well ... traveling! The other possible misperception is that you have to travel often to find premium travel cards useful enough or to be able to offset the AF. Some cards work well for the occasional traveler and you obviously do travel at least occasionally since you want to waive FTFs!
"I am only seeking cash back credit cards at the moment as I do not currently travel frequently enough to take advantage of point systems (yet you are asking for the) ... best cash back credit card that has no foreign transaction fees, is a Mastercard (so I have a Mastercard and Visa when traveling) 
So my suggestion would be to consider the Citi Premier card. It pays 3x TY points on Restaurants, supermarkets, gas stations, hotels, and flights. It has no FTF. It's a Mastercard. It has a $95 AF but there is a credit that could more than offset the complete amount on a multi-night hotel stay or one night at a high end or resort hotel. You get a $100 credit against a single hotel stay costing $500 or more. Vacation, anyone? TY Points are worth more if you redeem them for travel, but you can cash them in also. They can be redeemed on Amazon at 8/10 ccp. For travel, when transferred to partners, they can be worth up to 1.7 ccp. For statement credit, the cash value is only about 1/2 ccp, so not the best redemption. Still, for all the categories above you'd get a minimum 1.5% cash back for statement credit which beats the 1% of some other cards, especially if the hotel credit is useful for you. And you have the chance to transfer and earn as high as 5.1% with travel partners. In all, the Premier could be much more valuable to you than some other cards, even with the AF.
Here's a few other suggestions.
HSBC Cash Rewards is a Mastercard with no FTF. It pays 1.5% cash back but has a SUB that pays 3% back on the first $10K in purchases within the first 12 months. If you want a higher paying card and are willing to open banking, they waive fees on the Advance Mastercard.
https://www.us.hsbc.com/credit-cards/
Suntrust (soon to be a part of Truist with BB&T) has almost 3,000 locations in the eastern and southern US. I believe you have to be in geofence for their cards. They offer Mastercard products with no FTF. Their Cash Rewards card has a SUB that pays 5% back on $6K grocery and gas spend in first 12 months, then 2% on gas and groceries and 1% on everything else. They also offer their Travel Rewards and Prime Rewards cards.
https://www.suntrust.com/credit-cards
Barclay's USA has a lot of lower-end travel cards and many are Mastercards with no FTF. Some of these cards do have lower AFs (under $100) but they may also offer enough value-added to recoup all the cost, depending on how you use the cards. These include cruise lines like Carnival, Holland America, and Princess Cruises, airlines like American, Jet Blue, Hawaiian, Frontier, Emirates, and Lufthansa, and hotels like Wyndham and Choice. They also have cards branded by Priceline, Barnes and Noble, UPromise, and the NFL. Depending on the card you choose and the rewards offering, you might find something here that is worth more than a 2% card to you.
@cws-21 wrote:Aim_High, thanks a lot for the thoughtful response and recommendations. I didn't specify which rewards I am hoping to add to my current setup because I think I am pretty much covered based on my typical spend unless someone can identify something that I may be missing. You're correct, in that, the card would primarily be used for the minimal international travel that I will do. I know there are mixed feelings about Synchrony and the Synchrony PayPal Mastercard, but do you have any idea whether or not there would be an issues using it abroad? Regarding your comment about Capital One cards, are you suggesting that the Capital One Savor One may be issued as a Mastercard, but it could be issued as a Visa? I will definitely check out your other recommendations as well. Thanks again.
You're welcome, and I still think the Citi Premier would be an excellent choice, certainly worth a serious consideration. The main valuation questions over the other no-AF cards would be if you could use the hotel credit and how you would choose to redeem the points.
I don't have Synchrony cards so I can't really speak for other's experiences using it abroad. That's worth a separate thread to ask just that specific question, and I'm sure you'll get some great feedback. We have members who love it, but I'm just not that enthused about the PayPal Mastercard or Synchrony.
As for Capital One, I have a Quicksilver that is a Mastercard, but I'm pretty sure in the past that they've issued both Visa and Mastercard. Right now, if go to the Savor One page disclosure, though, it appears that the Savor would be a Mastercard. (The fine print says, "Mastercard is a registered trademark, and the circles design is a trademark of Mastercard International Incorporated.") However, I would verify that in the community or with Capital One before I committed to it if that's a major factor for you.
More than anything it sounds like the PayPal Mastercard and Capital One Savor One don't offer any real day-to-day advantage over the cards you already carry except for being Mastercards with no FTF.
The Elan Max Cash would have the FTF, but would be more useful in day-to-day spending. And if you're only using the card as a backup when traveling, paying an occassional FTF but saving lots of money month-to-month may still be a winning equation. One of the more popular 5% categories is home utilities and you can keep them all year long with a cap of $2K per quarter. Other areas include cellphone providers, TV-internet-Streaming services, Entertainment, Recreation, Department Stores, Furniture Stores, Electronics Stores and Fast Food. You can choose two of those with a $2K quarterly cap. And also another category at 2%. Everything else is 1%. If you can optimize the 5% at $2K quarterly spend, that is $400 in cash back annually which would cover the FTFs (at 3%) on almost $13,500 in spending!
Here's a link to the rewards summary:
(*Yes, it says VISA but because this was a Visa version of the card)
https://card.myaccountaccess.com/credit/elanmicro/pdfs/categories_max_cash_preferred_visa.pdf