This is the design for eyepiece caps, to stop the aperture at the exit pupil

Can be used on a compound microscope to get "stereo" effect, by putting the half- stop (or half-moon)

It is customizable, to fit different eyepieces. If the Thingiverse Customizer doesn't work, well, install and use OpenSCAD!

I've put one ready made STL for eyepieces I have, maybe.

You need to measure your eyepiece diameter and the Distance of the exit pupil. To get this distance, turn on the illumination of the microscope; take a piece of white paper and hover it in front of the eyepiece. When the illuminated disk is smallest, here is your exit pupil. Try to measure how far it is from the eyepiece. Usually is about 10mm. See last image for example.

Print setting whatever; does not need support Please black material, not fancy blue! It clips to the eyepiece barrel; may need some tweaks to get the right diameter.

For best, sharp half stop, recommend to make the half-stop slightly off-center ("moon phase"= 0.3) then trim it with a knife.

Yes it is a simple and crude design, but woks, may be inspiration for more refined caps.

Edit update 1.1 Improved design , nicer corners