Oil Cap loose

I've discovered that the oil filler cap on my 1946 415-C C-85 is coming loose in flight. My mechanics say the cap and filler neck look good and it feels tight but doesn't "click" into place when tightened. It's an "Aero Fabricators” cap/ dipstick.

Thoughts/solutions?

Thanks!

Brandon
NC2448H
Wallingford, PA