The one thing you should know about insulation is that it doesn't care which way you are trying keep the temperature. So... think pot cozy! I think your best material would be the reflective window screens. would put the reflective stuff outward facing and make sure the container seals well.

If you are looking for something ready made, meritline sells small cheap lightweight coolers: http://www.meritline.com/insulated-cooler-lunch-bag---p-43503.aspx

Just don't expect them to last forever. I had the strap rip out on mine.

Ohhh... don't worry about the lightweight snobs. Many people reduce pack weight so that they can bring more luxury items along. I personally would never want to hike a can of soda up a mountain (but I don't drink at that bottom of the mountain either), but some sausage and eggs sounds pretty tasty.