I think I found the ideal gasket material to use when modding the petcock to be manually (instead of automatically (using vacuum)) selectable.
Go to:
McMaster-CarrEnter "viton" into the search box.
Click on the "Viton Flouroelastomer Rubber" link.
Click on the "Sheets, Bars, and Strips" link.
From there, you can choose your options.
It looks like you can get 6" square sheets of standard Viton rubber for $6.29 (1/32" thick), $9.57 (1/16" thick), or $14.24 (3/32" thick).
In order to make shipping costs reasonable, you may want to "fill out" the order with other goodies (never a problem when shopping at McMaster-Carr).
IHTH someone!