I would investigate in the automotive sector. Think of the gasket that seals a diesel or petrol fuelled cylinder. Several special materials and technologies have been developed in the past.

On Google, you may find companies like this one:

https://www.henniggasket.com/high-temperature-gaskets

If this is not enough, try the aero-space industry (rockets).

I think that at this temperature you need use a metal gasket, maybe copper, silver, Cu-Be, or something like this.

Best regards

Pedro