Parametric OpenSCAD-script for telescope eyepieces.

PETG or even PLA should be fine. I'd recommend a diameter 1.5 mm greater than actual eyepiece and 3 to 5 bumps that, if needed, can be sanded down to get a good fit. 

I've provided .stl files for the eyepieces in the main photo. They are made to be printed using a 0.4 nozzle at 0.20 layers with 0.45 extrusion width, which should make for just four continous walls.