Full plate 'film holders' can be difficult to find. Most are 'plate holders.' I wouldn't worry too much about full plate until you get a few holders found. Could be a while. Or you could take up wet plate collodion and use the plate holders. I have an old Ansco 8X10 to 4X5 reducing back that would be a good place to start for the 4X5. You'd have to get a cabinet shop to trim it to your dimension and router in the drop in light trap area to match your camera. $60 if you want it.