Bought my chrome one direct from Billet 6 years ago for my 2011. Transferred it to my 2015.

 

Mercedes used to do this best. Our long-since gone 1995 E320 had a steel fuel door almost the size of the Super Duty door. On the end of the door opposite the hinge was a metal loop attached to the door, but on the inside. When you locked the car, a lock actuator would push a steel pin through the loop in the fuel door...it could no longer be opened. Would a pry-bar open it...sure...but it added just enough security to thwart most thieves or evil-doers as "W" Bush would say. Unlocking the power locks would cause the pin to move back. Infinitely more convenient than having to use a key at each refuel opportunity. There was also an access in the truck where you could manually move the pin if the actuator ever failed.

I think about locking fuel caps as a way to prevent theft of fuel. In the Northeast where I live, diesel fuel is sometimes more compelling to steal than gasoline, because it can be used in your furnace to heat your home. Diesel pickups tend to have higher capacity fuel tanks and thus greater reward. You would not believe the number of people filling up 5 gallon cans with off-road at the gas station because they cannot afford to have an oil delivery truck bring the minimum 100 gallons.

Personally, I have not invested in a locking cap since in my area the normal method of theft is a cordless drill with a 1.5" drill bit. The thieves are through the plastic tanks quickly and they fill up as many 5-gallon containers as they can. Messy, but fast, and you can hide under the vehicle.

This thread makes me think, though...because the modern diesel pickup is quite the target for ruining the engine. Contaminated fuel with water or some other harmful substance would result in a massive payout by the owner. Pouring something wrong into the DEF tank would also cause problems, but nowhere near the magnitude of expense on the engine side.

I'm thinking a locking cap might be in order, at least for the fuel.