Maybe one of our Great mechanics can come up with something.
This type will fit the older trucks but the newer ones are cap less. A company would need to design one that could mate to the cap less design and also lock. The def has a threaded cap but is much smaller. I think eventually some company will make them.
If I recall, the "official" reason was so that the fuel and DEF openings could fit right next to each other (i.e. an actual fuel cap is larger in diameter). If this is true however, I'm not sure why they couldn't make the entire fuel "cubby" (or whatever you want to call it) slightly bigger?
Do Fords have the DEF fill right next to the Diesel like our Rams?
My son-in-law has a 2015 Ford diesel. It has a large fuel opening with a cap like we used to have. It will take a locking cap.