When I was making and selling my XS650 ThermoDipSticks, I made gaskets using both 1/16" and 3/32" cork-rubber gasket material, and both have worked fine and held up with no problems. I used a special punch that created the gasket in a single step, but if you're just making one, then you can cut the O.D. out with scissors and cut the I.D. with an Xacto knife or a razor as Yamadude suggested (cut the I.D. first).
A thin coating of something like Yamabond is required to hold the gasket in place.
I no longer have any ThermoDipSticks, but I still have several gaskets. If you want to go to the trouble of sending me a SASE, I'll send you one.