Thermoswitch Gasket?
 

Friend's Miata's thermoswitch was dead so we put mine on his car, and I left it on, because the Miata's a 2nd car for me. However, we had some nice weather and I'm itching to drive it. So I ordered a thermoswitch from NAPA, and it didn't come with any gasket to help seal the switch to the themostat housing, and is causing it to leak. What should I use for a gasket/seal?

a little RTV should do it

olderguy you are my hero!