Black Fox, i agree with you (unless you have a vert. locking keeps lookie-loos out).
i did not know they were an option either (then n/a). if you convert, make sure to use the cable with it (cable goes into trunk, for when motor fails). mine is finicky and unless the battery is topped off (i dont drive the car much, but work on it a lot, draining the battery), it wont work. might have some resistance in the wiring.
you can hook it up to your alarm (just like you do the trunk release), so you can leave the glove box locked, if desired.
as for a locking cap, i think its a decent idea. the fuel doors are not strong. if someone wants in to take gas or dump something in the tank, its not an obstacle. i bet parts stores sell them. there arent that many sizes of necks and threads, since the neck is mandated by law to be a certain size (going back to when leaded fuel was around-werent supposed to get a leaded nozzle into an unleaded filler). ford probably only uses a few (or less) and since some of the other cars dont have the filler that secures, they probably see locking caps often. stant makes the ones in the parts stores here. try their site? if they have one.
good luck. let us know what and where you find.