DefenderWorx makes a
locking gas cap. I just put it and the matching trim ring on this morning. If you just installed the gas cap, the only tools you would need would be a pair of scissors to cut the leash on the original gas cap.
Unfortunately, they only make it in chrome but you could always hide it behind a black filler door. Wouldn't that give an eco-freak a nice surprise? Working to pop off the locked filler door only to find a locked gas cap waiting inside.