I have a 21 XLT with the 2.7 with one of those flapper type fuel fillers. I would like to put a locking gas cap on it for protection as it is parked out of my view frequently. I checked with a couple of dealers and they could not find one. You never know what you get from aftermarket on the internet so if someone has had a satisfactory result with this any help would be appreciated.
Edit: I’m more concerned someone will put something nefarious down the filler neck than someone stealing the fuel. I should have provided more information in the original post.

 

They can't be siphoned.

https://amzn.to/3PLXVor

 

Yep, the guys looking to steal gas are punching/drilling holes in the tanks because I believe all tanks with or without caps can't be siphoned. I've had capless tanks since 2008....non-issue

 

Your truck is most likely lifted for easy access to the plastic fuel tank, a quick punch hole and your fuel is gone.

 

Added David Hineline, to rain on everyone’s parade…

 

Living in Nefarious Cali, one needs all the help one can get, so the Ford locking fuel tank cap makes some sense(BU5Z-9C268-B Ford Parts for $44.95)...for the "Ford gas tanks can't be syphoned,"crowd....take a look at the YouTube tutorials on how to syphon the new Ford gas tanks using a bit of narrow poly tube and a pen-knife...I bought 2 of the gas caps 2 years ago(about $20 then)...planning for the inevitable...AND some POLY TUBING from HD!(be prepared!)....Ol' Rattler moving on, now.....