Looking for help finding or making a sheet film holder

My dad has a large format camera kept away and I'm looking for the film holder that fits this type of camera. The shape is square shape and is roughly a 8" × 8.5" holder with a indent of 7" × 7".

If theirs anyone who knows where I can find a cheap one of this size or a simple video on how to make one, I would greatly appreciate it. Thanks

The brand is Agfa and a link to show how it looks.