I get the best result with AN glass. I have tried Denglas which is recommended by Ctein..not very effective for me.
For 6x7 and smaller negatives one may buy AN slide mounts in 85mm square.
You will get 20 pieces on AN glass and 20 pieces of regular glass.
One method that I have found to be effective with regular glass is to tape the long top edge to either an open frame or bottm glass and then apply tape to the opposite side and trim it to the negative width w/o overhang. Then I Tape a piece of regular glass over this. The glass pushes the film flat and since the glass is resting on the tape the newton rings go away. This is fairly effective. Sometimes I still need to use AN glass. As far as the Denglass I do not recommend it.