Inside the shell, when you pull the insert out, there is just barely enough room to tape a filter in place between the felt on my 120J back. The same can be done with the square format shells but the room for taping is a bit dicier. Only drawback I can see is the filter needs a tight fit against the insides of the film rails to keep white light from leaking around. I don't see a good way to get it under the edge of the rails. As far as taping it to the outside of the back I see the fit is very snug, hard to tell if it will interfere with the operation any. I'd try a very thin polyester filter there with no tape. It will not slip there and I think the extra thickness of the tape may pose a problem though I could be wrong on that point.
Originally Posted by Soeren