If you can find some soft sheet copper the right gauge, another option might be to fabricate correct-thickness copper gaskets for the inboard exhaust ports- - - -in case the double-gasket arrangement doesn't work out.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!