This 3D model is an dew shield for various 50mm guide scopes / finders. Mine is from SVbony, other manufacturers such as Omegon or Orion offer the guide scope under their name.

Inside the dew shield, there is a small bulge. This prevents the dew shield from being pulled too far.

Tip:
For better grip, a piece of felt may need to be glued in.
When you have issues with reflections you may need to cover the inside of the dewshield with some felt or some antireflection color.

Use PETG or “better”.