Someone asked me about spatial filters and this idea came to me. What do you think?
pass beam through first lens, pass through spatial filter, put a mirror just behind the second mirror, bounce the beam back to a second mirror in front of the first mirror and out through the second lens. You could put the spatial filter where ever in the path that makes sense. Sounds like a pain to align but can't see why it would not work. The idea is to get more path length in the same space. You could even go sideways and back or such. First lens would be on an angle so you still use lens center.
fact is the more bounces the more loss and the more to go out of alignment but still cool to think about.