Tore out my old crumbling seal last year after it started leaking heat bad. Went with a thick ceramic fiber rope gasket from a general furnace repair site, not even pottery specific. Stuff was cheap and I just used high temp silicone to glue it in place. That combo has survived maybe two dozen firings up to cone 6 so far without any shrinkage. What kind of temps are you usually firing to?
Hold up, using general furnace gasket material sounds risky to me. That stuff isn't made for the repeated thermal cycling a kiln goes through. I've seen it get brittle and flake apart after a while, leaving little fibers everywhere. Pottery specific products are tested for that exact environment. Saving a few bucks isn't worth the chance of a failed seal mid firing.