The filter set I have is for use above the lens, but my Gmome II Beta has no filter slots. Should I put them in the negative holder, underneath the negatives? Alternatively, I could cut them into small squares and place them at the bottom of the inside of the bellows, so they'd be a few millimeters above the lens.
How far above the lens should the filters be in order to get the best results?
I would start by just laying them on top of the negative carrier. If they are in good shape there should be few problems. Of course higher is better to avoid focusing on any imperfections in the filters.
Keep an eye out for a used set of below the lens filters. It seems as though someone is always getting rid darkroom stuff so you might get a set for free.