Bzzzt. I'm sorry, those answers are incorrect.
The answer is "Garage door insulation"!
I got R3 foil lined bubble core in the exact width to be installed in the door without any adhesives or fasteners, just tuck into the door channels. 50' roll is about $50. The lined panels are now 31° F cooler than the unlined ones with the door facing the blazing sun on a 97° day.
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