Focal Point (Mike Sparks) is the BEST source for anit-newton ring glass. Many people say 1 piece on top is enought, but I would recommend useing 2 pieces, I have had too many problems that will drive you crazy when I only use 1.
Here is a negative carrier I made, which I think works very well. I made it out of 3/16 gator, but have also made them from wood in the past. The gator was a quick and dirty replacement that has turned out to work better than I had expected. The glass is actually for a devere enlarger and is about 5X7 in size.
The carrier is about 2 inches larger on each edge than the glass. But the back side which is closest to the column needs to fitted to your specific enlarger. I say this because if you make it too big you won't be able to center the neg in the lens.
I made it buy cutting 2 pieces of gator to the same size which is the outer dimension of the carrier.
Then, I cut a window to about 4X5 inches in one of the boards. You can cut smaller if you want to make a smaller carrier or to make sure you don't have any light spilll over. However I cut mine slightly larger and use tape to cover any light spill over.
The second board I cut to the size of the glass and then a few cut outs for my fingers to life the glass as I need to clean the transaprency after it is in glass(don't underestimate your need for the cut outs!)
Then I put packing tape over all the edges I've cut so there isn't a dust problem and tape the two pieces together!
Make sure to use a think enough board so that the glass fits completely into the whole it fits into... otherwise you would some light spill over where the head meats the carrier.
These carriers work very well for me, I use the EVERY DAY, and I've used this carrier for years!
I hope this helps!
Corey