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.
