Rheinberg filters can be easily made by printing the filter on heat resistant overhead transparency foil and then cutting these out. The center of the filter should have a different color than the periphery and should be significantly darker. You can obtain the desired color density by stacking together several filters. Be aware that the filters can trap quite a bit of heat (use heat-resistant foils). The heat can also make it more likely for the microscope lamp to burn out (LEDs are less sensitive).