For my 35mm and 6x7 negatives, I use a glass sheet taken from a glass slide holder. The slide holders I have were made by GEPE and come in an anti-newton version. The glass slide holders contain two pieces, one is normal clear glass and the other has a fine texture to miminize newton rings. You can use either one according to your needs. The glass pieces are very thin (less than 1mm) so they don't interfere with the operation of my normal non-glass negative carriers, but are large enough for the holder to clamp down on the edges. I simply place a piece on top of my negative and close the holder normally. This has markedly improved the flatness of the negatives and provides sharpness all the way out to the edges. Hope this helps.
All the best,
Daniel