I recently completed a project scanning a large number of old poorly stored negatives which had been kept wound way too tightly or left loose in an open cigar box for 40 years in the basement. There were lots of odd sizes (126, 127, 828, and more). The problem was that the film holders for my Epson 4990 weren't the right size OR didn't have the strength to hold the negatives flat.
I purchased two 8x10 sized pieces of 'museum glass' from a framing shop. This is glass with an anti-Newton's rings style surface on one side. Unlike real anti-Newton's rings glass, this material gets you 90% of the results for only 10% of the cost. I think I paid about $15 for both pieces.
The glass, as cut, has sharp edges and needs to be sanded down or taped off to prevent scratching the scanner platen. I used blue painters tape and covered all of the edges. I then used the tape to tightly affix my mangled negatives, emulsion side away from the glass, to the anti-Newton side of the glass sheet. Using the tape allowed me to tape the edges entirely. This accounted for any direction of bend or curl and kept everything very flat.
Through experimentation I found that placing shims in the four corners of the glass were necessary to prevent Newton's rings on the scanner platen to film interface. Also, my Epson 4990's optimal focus is slightly above the glass surface. I made my shims using squares of the painter's tape. It is easy to add or remove pieces to get the right height. I found 30 strips, stacked, in all four corners worked perfectly.
The second sheet of glass was prepared similarly. The second sheet allowed me to mount the next set of slides while the first set was scanning.
After scanning was complete, I removed the tape holding the negatives and returned them to the cigar box (the point of scanning is that I never need worry about these negatives again!). Then I removed any fingerprints, dust, or adhesive using a quick lens cleaner spray, wipe, and then the StaticMaster brush. The glass was ready for new negatives.
Using this process I was able to scan about 4000 old photos with resolution that far exceeded the low-end cameras on which they were originally shot.