Since the illumination unit built into the cover is 3.25 inches wide, why limit myself to the 2.25 holder. The film in either the 35mm or 120 holder sits on a surface that is 0.020 inches above the glass. So I bought a few sheets of craft plastic, through Amazon. ( Then traced the supplied holder onto the plastic sheet. The plastic is clear and covered on both surfaces by a removeable protective sheet. I cut out the window with an X acto knife. I made a few different holders, (25 12x12 in. sheets in package) . Lot's of room for experimenting.
I'm quite pleased with the results. Many but not all my 4x5 are suitable due to image size. One could make a custom holder for 6x12 or 6x17. The size of my "window opening" that works best for me is 3 x4.5 inches.
I know that this is an old scanner but the concept should work on newer ones. I use black art paper to mask out all but image area. Have fun!
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