[BotaBigot]

Existing fan just might reverse. Worth a try with a quarter-inch ratchet. Put cardboard against radiator to protect fins. If it works, this would be elegant in its simplicity.

I think your "cookie sheet" would totally restrict airflow and overheat.

Playing devil's advocate...if you would go to the trouble to install microprocessor coolers, why not just put in the heater core...