Guys,
I don't remember who asked but someone wanted a suggestion for sealing his ductwork to the radiator. I happened across this material at Spruce while perusing the catalog the other day.
Silicone Airbox & Heater sealer. AS part # 05-00662.
A soft silicone mat with a fiberglas reinforcement.
Sounds like the perfect material to me.
Bill Jepson