I like simple solutions to printing problems whenever possible. After lots of testing I'd like to offer this one to anybody who has ever had problems with newton rings when enlarging with glass carriers.
Simply fix and wash an unexposed, undeveloped sheet of Tri-X 320. Because it is designed for retouching, the base side is just rough enough to prevent newton rings when pressed against regular glass. Place the clear Tri-X on top of your negative and sandwich the ensemble in the glass carrier. I treat the Tri-X sheet with the same care as my negatives so dust and scratches are not problems. Light loss through the sheet of Tri-X is 1/2 stop.
I prefer this to the use of anti-newton ring glass which can be problematic particularly with condenser systems, or with uniform areas (such as grey skies), or when enlarging small negatives.
This works with all formats. I've been using it with sheet negatives and 35mm negatives with success. To size the Tri-X spacer for any format, simply cut the sheet down to size.
Tri-X also makes a good surface for selective masking. Light loss is less than with most frosted materials such as matt Dura-Lar, and is more uniform as well.