I'll second the cooler idea.
Last year a few of us went to Cedar Point Amusement Park for the day. I'll shell out the money for UL Gear, but refuse to spend $70 on overpriced food and drinks per person.
So, I made a cooler out of left over Reflectix to keep food and drinks in the car.
It worked well enough. I kept the cooler in the trunk. It was sunny and the temp got up to over 85 that day.
By 7 P.M. when I made dinner, most of the frozen items had thawed about 90% but were colder than a refrigerator would keep them.
If you want it to last, I'd suggest covering it with material though. Alone, I've found it not as durable as I'd like.
As far as the dual purpose cozy goes, I made one from breathable material and Insul-brite that works great as a cozy and as a stuff sack for my cookset, and is lighter than one made from Reflectix.
One other good use I found for Reflectix is making a heat resistant outer case for my sunglasses. I have a pair in my car year round, and the intense heat in summer and cold in winter always caused stress cracks in the lenses. Expansion and contraction I assume.
I made an outer case from Reflectix to keep them in when in the car, and haven't had any cracks appear yet, and it's been about 2 years.