What I saw came back to mind. I saw several made by Mark Osterman. He cut out the septum and placed small corner supports in the holder for the plate. The length of the cutout was shorter than a plate by a fraction, but the width was correct. The plate slid in and was supported by the 4 small corner supports. He used 2 different styles IIRC.
We talk about this sort of thing at lunch.
What will we do when they stop making the plastic holders? And, 4x5 cameras?