I'd like that too. I had one on my 2011 Taurus but didn't fit the Mustang.

 

I'd like a locking solution too. The filler door doesn't even have an inside release on my 2018 GT. Anyone can walk up and just open the fuel door. Vandalism is as much a concern as gas theft, and potentially far more expensive!

I've checked accessories.ford.com and Amazon, americanmuscle.com, AutoZone and so on. Nothing. Locking plugs are only available up to MY 2014 (Part #8U5Z-9C268-B). This seems to be a device that is available for so many other cars but not ours.

And I paid $395 for the Enhanced Security Package option that even includes wheel locks but I can't secure my fuel door!

I'd pay money for a lock for the door or gas cap.

 

Any solution found yet? I’m considering custom fabrication at this point.

 

This one worked, 2018 Mustang. It is marketed for a different Ford, forget which. Works flawlessly for me, but took a few tries to get fast with it.