Problem: need to linear polarize excitation light path of a Nikon Ti.
Problem: Nikon's slider parts have empty holes that are too small for a filter and the other positions have ND filters that are epoxied in place.
Solution: use cardboard or foamcore board cut with a razor blade.
Results:
Yes, we know the filters need to be cleaned!
Left two panels show the polarizing filter with the orientation the same as in the exciatation light path. The red light is a bright spot on the ceiling.
Right two panels show filter rotated 90 degrees and light blocked from projection on paper taped to ceiling.