Our neighbors on the distilling side of the street use cork as the gasket due to concerns about chemicals being leeched into the "brew". Of course vaporized alcohol is passing by their stuff, but it still might be the answer for you. You can by rolls of the stuff for cheap and cut to your size.
http://homedistiller.org/forum/viewtopic.php?f=33&t=13514
http://homedistiller.org/forum/viewtopic.php?f=33&t=13514