How do I seal the Choke Thermostat?

I just got my '71 Duster with a /6 running last week and I am still working thru a few minor issues. I have a '72 exhaust manifold on the engine which has the thru hole and a stamped steel cup for the carburator thermostat. I have found an original carb. thermostat which I will be adding but the gasket is not available. I already tried making my own gasket out of a sheet of collector flange type material and it lasted one day. The permatex ultra copper high heat silicone worked much better for around 100 miles. I was thinking about trying the permatex high temperature metal repair compound.

Has anyone tried this as a fix, and if not what is my best option (other then changing the exhaust manifold since I would like to keep it period correct for my '71).

Any ideas would be appreciated, since I am sure I am not the first person to have this problem.

Regards,

-Matt