Originally Posted by Fabrik8

You'll have to take a look at some datasheets, or general properties if you can find them somewhere. You're just looking for something with low thermal conductivity and not a lot of thermal expansion. You may want to shoot for a plastic that is similar to the coeff of expansion of aluminum. After that, make sure you factor in the cost. No one will want to buy a thermal gasket that costs a lot.. It's great that you have access to a waterjet, you can't get any luckier than that for making a lot of car parts.. If you have a digitizer it will really speed things up, the gasket doesn't have to be perfect, the only thing that matters is the ports and the bolt pattern, so a lowres digitization is plenty if you're going to clean it up in Solidworks. Good luck!
If you're wanting to make money with gaskets and stuff, you may want to think about making turbo flanges and exhaust (head) flanges for turbo setups. You should be able to sell a lot of those locally if you get your name spread around.

i'm jealous, I'd love to have a waterjet for trimming composites parts..