As I'm sitting here pouring my coke into a styrofoam cup, it occurs to me that styrofoam is a really great insulator.
I don't know if you can buy the stuff in rolls, or if you would have to carry around the large peanuts of it, or if it would be durable at all, but it would certainly be light weight.
Anyone ever thought of trying to configure some sort of underside insulation using some type of styrofoam?