If you could get away with a thinner material how about ringing you local tyre mart and scrounging a scrap inner tube. As long as its 2 inch or larger diameter you can probably get the sheet size you need out of it.
I very much doubt it. If the surfaces are in good condition then generally the thinner the gasket the better. Rubber tends to distort and squeeze out of the gap, particularly if overtightened. A smear of ordinary clear silicone sealant from your local diy shop will prevent any leaks. In fact you could just use that without a gasket. Apply a thin bead, nip the cover up lightly, leave a few hours for the silicone to cure and harden and then tighten a tad more.
-- Dave Baker Puma Race Engines
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Camp USA engineer minces about for high performance specialist (4,4,7)
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