That is a sweet solution to an old problem!
Originally Posted by matti
Long ago metal septums were available to hold film in plate holders. These were usually made of metal with three edges rolled over to retain the film. They can be improvesed from thin sheet metal and painted black. There will be a slight focus shift, about the thickness of the metal. The typical small apertures one might use with an old plate camera should cover that. The film should be backed with loose cardboard to minimize this. With a little more work, one could adapt 5x7 sheet film to whole plate holders.