Plate holders are deeper to accommodate the thickness of the glass and you'd have to fill that space with something the same thickness as the glass plate it was intended for. If you didn't, the film would not be at the expected film plane, and your images would be out of focus. But, more of a problem is that the holder will not have any edge slots to hold the sheet film flat and in place. You might be able to construct something that will work out of thin plastic, but I think it would be easier to just buy a regular film holder.