Cherokee Strut Access Plugs

We have been having trouble getting decent replacements for the above, which
pop into the holes in the top of the wing to allow the main gear struts to
be pressurized. Our mechanic uses 1 1/2" plastic (Polypropylene, I think)
hole plugs from McMaster Carr, but they don't seem to stay in our wing.
We've lost two over the past couple of years. When we lost the first of
our original Piper ones, we got a replacement from Wentworth. While we were
waiting for that to arrive, we put one of the plastic plugs in as a
temporary measure. It was gone on the next flight. The replacement from
Wentworth turned out to be a well used (i.e. rusty) steel knockout cover
from some electrical supply. I painted it, and it seemed to work OK. The
other day, that disappeared. Back to the black plastic plugs. Gone again
on the next flight. These seem to work well in other Cherokees, including
the school's rental planes, but for some reason, they don't stay in ours.

I found some better quality plastic (Nylon) from Heyco, and got some black
samples (the minimum order for the white "standard" color ones was 26,000!).
These seemed promising, but are just a bit too deep to lock in securely.
Has anyone found an acceptable substitute for the Piper part? I may have to
go back to the steel knockout plugs, or machine my own design (I have a
lathe and milling machine, fortunately).

--
Bob (Chief Pilot, White Knuckle Airways)