I designed this to build a cross-polarized setup for photogrammetry: polarizing film is held in front of flashguns, polarizing filter on the lens has perpendicular direction to cut direct reflections. It can be also used with color filters for artistic effects or for macrophotography.

Designed for Yongnuo 560, but you can easily customize the clamps to fit any other flashes, as the model is parametric. You may also want to change the lengths of the arms to fit your needs.

You will need a ¼"x1" tripod screw, 12 M4 nuts, 4 M4x8~10 screws to hold the filters and 4 M4x100+ screws to clamp the flashes. I suggest to make long weirdly-sized screws by cutting threaded rod to size and counter-tightening two nuts on one of the ends.