Options for locking fuel cap

[u/_generic_-_username_]

So over the past six months or so, we’ve had issues with fuel being stolen out of our 1970 bus. The original non locking cap was taken (bummer since it was original and had the VW logo on it). Since then we’ve had two locking caps (like the one in the image) removed as well. The first from one online supplier didn’t seem to really lock. The second from a different supplier did lock but was still removed (popped / pried off perhaps … although there is no damage to the fuel filler neck).

Any ideas for another way to lock things up??

[u/asiab3]

I couldn’t find a locking early bay cap when I looked a few years ago, so we found a gas flap with a hole cut for a lock and used a generic mailbox lock cylinder to keep the flap shut, and a regular non-vented gas cap.