[hellosailor]

jb, the art of cutting gasket material is a lost art. But the process still works as well as it always did.

Gasket material (the correct material & thickness for the purpose) or even a product like Permatex "Form A Gasket" should be perfectly good, as long as it is the right thickness, and suitable for use in coolant. Probably a lot cheaper and faster than ordering gen-you-whine marine gaskets, as long as you don't mind cutting it to match.

 

[hellosailor]

And if all else fails, usually NAPA stores have a better selection of "real" auto supplies like that. Inner tube rubber used to be good stuff, but I have no idea what it actually is these days, and it is often not flat.

Grainger and McMaster, both online and with nationwide stores, carry these type of materials in a very wide range for industrial customers.