1964 fresh water tank fill cap lock

[Safari64]

Has anyone devised a way to "lock" the fresh water tank fill cap on a mid 60's Airstream.

Short of installing a later type '70's fill door with key lock in exterior panel.

It's a simple 1.5" male plumbing bung type plug cap.

I leave my trailer unattended in the forest for days at a time, just want to keep/slow down unscrupulous types adding anything but fresh water in.
The ol' sugar in the gas tank "prankster" trick comes to mind.

Figuring one of you ingenious types have solved this before.

Thanks, Safari64