I have an older Zenith ML2000 compound microscope, and this has a photo-port for which I couldn't find any suitable sized phototube to attach a lens to. This adaptor slots into the Zenith photo-tube (the narrower end) and allows a wider lens to slot in the other (lens is 38mm to 28mm c-mount lens). scad used to generate it included so the size can be adjusted.

The part is printed upside down (wider part to bottom) to avoid needing support material.

The lens reduction power used depends on the size of the sensor being used - I used a x0.3 lens with a 1/4 inch sensor chip.

Photo of part in use and example photo (of pollen) from microscope.

Future improvement: add a screw hole so the lens can be clamped in.