I have this 35mm viewfinder which was made for the Nikon rangefinders. It's quite good and small, I can see the 35mm frames with glasses on, although the glasses touch the back of the viewfinder and could be scratched if precautions aren't taken. A piece of cut gaffer tape would solve that.
This pic of the rear shows there is good surface for some protection.

This pic of the rear shows there is good surface for some protection.
