Most sizes of Kodak "Cut Film Sheaths" only hold the film with bent-over lips on two sides. The long side, so far as I know. Then the film and sheath goes in the plate holder, where the plate was originally held on the short sides. So long as you make sure the film is registered where the front (emulsion side) of the plate was, there should be no reason for any focus error using a film sheath. Yes, the lips on the side will be forward of the film plane, but they should be out of the way of the dark slide on a typical wooden plate holder.