franco I've tried to find these on the web but they all seem to be threaded and made of hard plastic.
Hi Franco,
Are you looking for one of these?
https://agenaastro.com/agena-rubber-end-cap-2-50-8mm-black.html
I think this is the set I had at one time bought from Agena. They fit over the 2" flange of a ZWO hockey puck camera. Agena has all kinds of these end caps for different sizes; but be careful, some of them are hard plastic. The rubber ones are specially labelled as rubber.

If the camera's flange is not tall enough for a flter, you can always add a 5mm or 10mm M48 spacer to the camera. But the end cap fits fine with a typical 1.25" filter in the M48-1.25" adapter.
Chen