An ice chest lined with preheated bricks, or rocks, or sand, or uncooked rice, or etc.
Actually I prefer to leave them in the ice chest as not only can they make a nice bed to help stabilize the food containers as you nestle them into the gravel or whatever, but more importantly, the more mass you have, the more efficient your thermal holding capabilitiesYou use those materials just to preheat the
cold ice chest walls.
Same basic principal that a full freezer is more energy efficient than a half full one.Interesting, especially with sand, Ive never done it that way,
always took the preheat materials out and relied on the
thermal action of hot food in an enclosed environment.
I may try it that way sometime to see how it works.