I made mine out of a shet of compressed graphite, this is used for macking gaskets for high temperature flanges, I used one of 1mmthick and when torked down is aprox, 0.6mm. The advantage is that it seals perfectly and you get an increase of compression ratio free! The ones I made were installed about ten years ago and have not given any trouble since.
Enquire for this material at industrial hardware stores, the should be able to help.
Robert Lander