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.
1mm glass OVER TOP of film negative worked for me.
I've just taken a couple of images using paper negatives (Ilford Multigrade) in a plate glass holder. I used some black mount board cut to size and just laid the paper on top - no jam, no honey (although that does sound like a neat trick). This was the result (after inverting the image)...
Wow, that's nice!
How did you 'invert the image'? Do you mean chemically? Or did you contact print onto another sheet of paper?
I have to admit to cheating here - the neg was scanned in and inverted, not done chemically. That's my next step, when I can get physically get to my enlarger (all my woodworking gear is in front of it at the moment), I'm going to contact print it and see if I can approximate the image you see here.
So, watch this space for the full chemical treatment!