This a a Bahtinov mask and a holder for a 50mm Bigking Finderscope with a focal length of 190mm.

Two masks are included, one is a bit more open.

The outer diameter of the mask is 61mm and it might fit similar finderscopes from other brands.
It's designed it so that the holder and the mask are printed separately. That allows you to quickly change the mask for a different design without printing a different holder.

The mask itself is 1mm thick and prints in 16 to 20 minutes. I have used PTEG but PLA should be fine.

You just put the mask into the holder and then put both on to the finderscope. It's not a tight fit but the scope is probably pointing upwards so it shouldn't come off.

The mask itself has been created by this awesome tool: https://satakagi.github.io/tribahtinovWebApps/Bahtinov.html
If you want to design your own mask you just have to enter the focal length and the outer diameter. Please pay intention to the scaling - I had to upscale the SVG in Fusion360 by 1.33333 to get it to the correct dimensions.

This is my first model but I have many more to share. Comment and let me know what you think.

Clear skies!