Viton is best. You can get one foot square sheets in 1/32 and 1/16 inch thickness from smallparts.com. It's a little pricey, but it does the job!
Thanks. I checked smallparts.com but could not find flat sheet stock only viton o rings. What are the search words to find it.
Do a general search for "viton". When the results come up, on the left will be a product category list. It'll be under "rubber" in the material section. Hope this helps!
Thanks chaddilac. That is pricey for sure. well I did ask for the best. I wonder if there is any thing else that I could get by with less exspense. I need to make a flange gasket 7 3/4 inches square so 12"x12" sheet would work.