Just wanted to share this.
- I used a 12"x12" 1/8" viton sheet stock
- tacked sheet stock to a piece of plywood.
- used a 3 1/2 inch holes saw to mark center and edge. did not go all the way through.
- used a 2 inch hole saw to cut out the center
- finished cutting the outside with 3 1/2 inch hole saw.
Its slightly larger outside diameter than stock so I had to hand trim the outside edge. I didn't have a 3 1/4 inch hole saw, but that probably is the right size, and wouldn't require any trimming.