My refectrix cozies worked too well, in that my food stayed too hot for too long. Not fun when hungry, ask the guy with the blisters on the roof of his mouth. Like a sleeping bag, my winter cozy was too warm for summer use. Mine were cylindrical in shape to minimize material/weight. I'm now using a much thinner, cut off, bubble wrap mailing envelope. It is the right combination of weight and function. Away from bear country, it is part of my pillow stuffing.
Think about function before investing in optimal materials (the cuben, aerogel cozy), unless you are building a backcountry crock pot.