To use my Pard Nigth Stalker 4K scope without the rubber eye releive and to protect the lens I was checking Makerworld and run into the Parametric Rifle Scope Cap - Free 3D Print Model - MakerWorld. 

Had to resize it to fit the Pard scope. Also I made an adaptor in TPU to fit better.

I don't  glue it but you can. As a pin I use a bit of filament.

The inner diameter is 41mm