Not quite what I meant. I'll make a drawing. The idea is to food the path on itself to compress the length of the module. So you pass through the first lens the through the spatial filter the back toward the first lens, off a mirror in front of the first lens and out a second lens across from the first lens. The lens are offset a touch from each other to account for the mirrr path.

Other idea was through the first lens, through the spatial filter, off a mirror and back through the first lens but a touch off set. Means the spatial needs to be tall and thin.

I'll draw it. The idea it to let you use longer focal lens in a smaller space. So if you need 400mm and this could fit it in something just over 200mm.

You could stack the beam on top to but that would mean more mirrors.