CP 35-6HB (qty. 25 or 100)
CONTACT PROOFING PRESERVER. Holds six 35mm strips of 6 frames, and one 8-1/2" x 11" contact print.
CP 35-7HB (qty. 25 or 100)
CONTACT PROOFING PRESERVER. Holds seven 35mm strips of 5 frames, and one 8"x10" contact print.
for medium format:
CP 120-3HB (qty. 25 or 100)
CONTACT PROOFING PRESERVER. Holds fifteen frames of 6x4.5cm, twelve frames of 6x6cm, or nine frames of 6x7cm 120 film, and one 8"x10" contact print.
Unfortunately there is nothing like this for sheet film (though it is easy enough to slide precisely cut sheets of paper underneath the negs in individual plastine envelopes).
You are printing through plastic, of course, but the way I see it, they are just proofsheets. How good do they need to be, except on special occasions? They are more than good enough for what I need from a proofsheet, and certainly sharper than anything else I would bother to do under normal circumstances. Usually if I need a *truly* sharp proofsheet, I will go to the local rental darkroom or this one local gallery that does all their printing in the back, and load all the film into an 8x10 glass neg. carrier and make an enlarged proofsheet. A&I, the big pro lab in Los Angeles, can do this for you as well.