The stereo zoom microscope I recently bought came with 30 mm diameter eyepieces. I also bought another set of eyepieces but these were 23mm diameter.

So I designed the attached converter so that the 23 mm diameter eyepieces would fit in the microscope.

There will be shrinkage due to the nature of whatever filament is used - mine were made with ABS. The outer diameter fit the microscope well, the inner diameter had to be sand papered to fit. The bonus is that sand papering made an exact snug fit on the 23mm diameter eyepieces.

I rolled up a piece of coarse sandpaper into a cylinder and used that to slightly widen the inner diameter for the snug fit.