padraigm, I apparently live in a world with different physical laws than everyone else.
I started platinum printing trying to use an 11x14 contact printing frame and could never get tight enough contact until I started jamming pieces of card behind my paper to force it to be tighter. I finally gave up when I started getting little wood chips to deal with.
I decided to just use 1/4" plate glass, two sheets. I found I needed clamps to really get it tight. The glass was 11x14.
Then I started printing on 11x14 paper and needed oversized glass. I tried 16x20 glass with 4 clamps and could never get it tight enough. I tried foam behind the paper and card or mat board behind the paper and could not get it to work. Very small areas would be out of focus. So I got glass cut 14x18 and started using 8 small size industrial clamps. 3 on each side and 1 on each end. Now I get tight registration. I have never seen any newton rings.
Awhile back I was hired to print some antique 11x14 glass negs in platinum. I needed over size paper to print on so I went back to the 16x20 glass and sandwiched the 11x14 glass negs between 2 sheets. A couple of the glass negs were not perfectly flat and some small areas printed a little soft.. I should have gotten a vacuum easel.
Perhaps you should consider a vacuum easel. You can make one pretty easily.