I did some crude testing of cozy material in my kitchen. I boiled 2C of water on the stove in my pot, then slipped it into a custom-made reflectix cozy, with lid, stuck a candy thermometer in the water and waited an hour. Then repeated the process with a fabric cozy, made of an oven-mitt material that JoAnn fabric calls insul-fleece, double thick.
starting with water at approx 210F (brought to boil on stove and then moved off)
Reflectix- @60 min: ~175F
Fabric- @60min: ~158F
I believe that the reflection does more than the insulation, but it's hard to test non-reflective reflectix to see, I guess I could just get some bubble-wrap and see.
Thinner car-shield stuff should work, I think probably as well as fabric.
If you want to coast-cook things like pasta, I'd do reflectix, if you are trying to not burn your hands, and rehydrate FD food, anything will work.