Although the cap looks cute I wouldn't implement it for gameplay reasons. Instead I would try to find a solution where the ball can enter the hole from all sides, for example a closing gate/ spiral/ ... in the hole itself.
The cap shall open and close and rotate with the ball, but I found no mechanical solution until now. While I was on the treadmill today, I had a design in mind.. I will test it next. But thanks about your shared concerns! I have a very technical, analogous, mechanical style in mind, we'll see.
have you thought about using screen space reflections?
Yes, yes! I will definitely give it a try!
