Sadly, virtually nothing works. Cardboard, plexi, masonite or oatmeal. My last iteration of a fix was a plate of .060" aluminum shaped to the face dimension and backed with ceramic cloth. And that's WITH a Mocal
oil thermostat. (tried 2 actually-they both sucked).
At highway speeds, you lose temp fast. It normalizes in stop and go but dies on the highway.
Best bet-remove lines and cap the fittings. Then reopen when the ambient is good or when tracking.
Guessing cold
oil not good for the Brothers Paxton too huh??