I met a German chap in Austria, umpteen years ago, who was using one of those amazing Contarex Hologon cameras. He had a home made lens hood that covered the whole camera with a slot for a cinema size gelatin filter, so he could see the effect of the filter in the finder. The whole thing was attached in place of the camera handle, with a cable release in a press camera handle on the side.
Heath-Robinson and Rube Goldberg would have been hard pressed to invent better but he claimed it worked well 🙂