I've bought 3 holders for my 6.5x9 cm Avus. They are for glass plates and one of them has a plate in it. (Too bad I didn't know about, took it out in light.)
I know adapters exist to use these holders with sheet film. Never seen such an adapter in flesh, so maybe my next question is silly.

Would it work if I get the existing plate to a glass shop and let them cut two other such glass plates from glass of the same thicknes? I could, I hope so, fit the film above this plate in the holder.