I'm inexperienced with large format. There is a Voigländer Avus 9x12 for sale with 3 glass plate holders without the plate needed to insert film. And it seems they are hard to get. In the background in the picture is a film pack holder? Is there a way to use it for single film sheets?
9x12 film inserts are fairly easy to find, unlike the plate holders inserts are standard. I've sold my spares but found a box and a half of new Eastman Kodak inserts a few years ago.
I guess if you know what you're doing it might be possible to use a Film Pack holder for single sheets the issue is they aren't light tight as the pressure of the pack acts as the seal.