small gas cap for side aux. fuel tank - help needed

[dsy1]

I need to find a small gas cap for my side aux. fuel tank. The neck inside diameter is 1.25 inches but I can't find anything that small. This is a cam-style cap (two tabs fit into slots on the top of the neck and it twists 90 degrees to lock on). The neck that comes up from the tank has a rolled top with the two notches for the tabs on the cap, with 1.25" inside diameter and about 1.75" outside diameter.

So far I have been using a push-in rubber cap meant for an oil cap on a valve cover.

I have tried a twist-on 1.25" oil filler cap with tabs but the tabs extend in too deep so it just spins and won't engage to lock. I could maybe add a thick rubber washer to make it tight enough, but I would like to find something correct instead. Maybe a cap for a motorcycle or something small? Any ideas?