Use the "hot hands" heat packs, but don't place them directly on the bags. Use wet newspapers or towels, etc. to insulate them. Every year I drive back from Buffalo during the holidays (9-10 hrs.) with corals packed this way, and it works well. If you don't have a styrofoam box or cooler, insulate a regular box with towels, etc. and place the hot hands along the sides as well. If you get a chance, once in a while shake the hot hands and move them around the box, but it's not really necessary. Double bag your purchases in case you spring a leak because the water will get absorbed quickly.