Epson V500/600 Scanners

I use a V500 for cataloging my negatives but the negative holder they supply will not hold the strip of negs flat enough to ensure a good scan right over the entire field. I borrowed a V600 negative holder from a friend and that is just the same. Any bright ideas please.

http://www.betterscanning.com/

https://shop.lomography.com/en/digitaliza-35mm-scanning-mask

I have a V500 and ditched the holders years ago. I lay the negatives in position so they curl away from the glass (emulsion side down usually) and throw an 8x10 piece of non-reflective glass on top. Negatives are always flat, negligible (if any) loss of focus for my purposes. Never any Newton rings.

Example:

You need to be kind with Epson film holders. And check if two parts clicked together. I do this and I'm fine.
The only time I could tell it is not flat is if I'm getting newton rings.
And to not get newton rings and have negatives flat I'm holding my horses.
Negative stripes goes to the book first. And staying in the book until they are totally flat.

If my negatives are so curly that the epson holder won't hold them down I use the Better Scanning glass pieces that fit into the Epson holders and then I tape them down at the top and bottom. This usually gets things flat enough that I get good scans, or at least as good as the V500 will do with 35mm negatives.