[Farside]

In theory yes but the reality is the inserts have a lip which holds the film, they are very easy to use. I have a pattern for making them but I need to search where I saved it. Will find it Sunday if I get time and post it here.

Ian

Many thanks.
It's just occured to me that a single sheet of ally with lips expoxied flat along each long side will hold the film central and I presume the film tucks under the top and bottom overhangs of the holder. The combined thickness of the sheet+lips would be the same as a glass plate.