This is a cap not for the propane tank, but for the valve/nozzle collar that connects to the propane tank. I went looking for one of these and didn't find one so...

Tested it and it works. I printed it with PETG at 0.2 and no supports. Might need a little cleanup on the inside diameter if the PETG gets stringy.

Print it with the handle part flat on the bed.

Note: It may seem like the inner opening is off center, but that's just an optical illusion because the threads are not symmetrical.