Dutchwest Inner Top Gasket materal

[jh6u]

I've got a ~1991 vintage small Dutchwest stove, #2181. It was rebuilt three years ago, installing the new style two-part Inner Top with Web, which contains the mounting surface for the combustor and the combustor damper. I recently noticed way too much creosote in the chimney, & found that the gasket between the two parts of the Top had come out. I'm presuming this is the source of the combustion chamber leak allowing heated gas to bypass the combustor. Does anyone know what this gasket is supposed to be? Interam material? There's a ~1/8" gap between the pieces- it looks like there's significant movement between the parts when heated.

Thanks