At Home Depot, in the moving supplies, there are thin foam envelopes for protecting fragile items like glassware. Those envelopes might make a good cozy for freezer bag cooking. They cost $4-5 dollars for 10 of them. Dual use is insulation under your head, unless you are in bear country.
I've stopped using Reflectrix because it works too well--I have to wait too long for the food to cool down after it is through cooking. I am currently using bubble wrap envelopes because they can be purchased in a variety of sizes and thicknesses. My preference is the smaller, thinner ones that work well enough.