Antenna question

I use my stereo daily, but literally never listen to the radio, so I
took my non-power antenna mast off of the car. I noticed on another
miata that was missing its antenna that the antenna receiving threads
had become rusty.

I am looking for something to plug the hole with, hoping that someone
has found that perfectly-sized black rubber piece from something common
that fits.
I have also considered a black screw with a tiny rubber o-ring, so if
anyone knows the thread size off hand, that would be helpful.

Thanks,

Pat