Probably goin' to need a welded in neck.I think almost all bikes now except HD's have a locking cap keyed to the ignition key for that bike.

Take the tank into an auto parts store and see if they have anything that'll fit??

I wonder why they put locking gas caps on bikes?...
They don't hold enough gas to be worth stealing....and the fuel line is exposed, so why put a siphon in the tank anyway?

Vandalism?...plenty of other ways to hurt a bike...

If you figure a way to convert a stock cap to non-locking,...Post pics...
I want to do mine too.

The "Trips" have to be dragged in to open the cap, locking or not, its gonna need the TWist in the middle, since the whole cap doesnt rotate into place, it just pushes in. Some work could be done, gutting the cylinder, Ive been in there & its not hard. Putting a fake "Key" in & JB welding it in would be no problem at all. Just grinding the key to slip in & maybe even drilling into the keyslot in the cap to create room for the Toyota key you are gonna use, or qwhatever, just build your own "KNob". Gutting the cylinder is a piece of cake.

after disassembly of gas cap (two screws, plate, o-ring, etc.), look at the bottom of cylinder there is a spring-loaded, tumbler-like retainer.
Tricky but you need either a super small screwdriver, or dental pick to push the retainer in (even with the cylinder wall) while pushing the cylinder out (away from you).
After that point keep fingers pressed on sides so tumblers dont shoot out at ya.
Leaving retainer/tumbler in, remove the others, reinsert and reassemble.

Then you can use anything to turn it.

Or pay local locksmith 10 bucks or so to have them re-key to your existing key. LOL
Is late, or i would have googled some pics for ya.

Peace.

Keyed gas caps keep gas thieves at bay! Why, 12 or 15 bikes and I have refuled the old F-150.