There are two types of 'anti-flare/glare' glass sold by framing shops: one is coated with an anti-reflection coating just like a lens; the other is very lightly frosted and a bit like a soft-focus filter.
Either one will eliminate Newton's rings: the anti-reflection coating will reduce them because it won't reflect light back from the film surface and so there is much less constructive/destructive inteference; the lightly frosted glass will break them up because the distance from the glass to the film is always changing rapidly - for light a change in distance of 500 nanometers is considered monstrous. You only need the treatment/coating on the side that presses against the negative.
Another possibility that just came to mind, and I haven't tried it so caveat emptor and all that, is frosted Mylar. It comes frosted on both sides and if placed between the negative and the glass should completely eliminate the rings. The frosting is exceedingly fine and won't show as texture on the final print. A blueprint shop or art supply house should have/get some, an 11x17 sheet should cost no more than a few dollars.