I use the modern equivalent of a hay box. I made it myself based on the brand Wonderbag. I would not pay £50 for some material stuffed with polystyrene balls. You prepare your food, liquid based like soup or stew simmer for a few minutes then nestle it in the bag. It will cook in 6 hrs or so. It will sit in the back of the car and be ready when we get to camp. The earliest that I have heard this method used was by the immigrants in wagon trains who used a dutch oven and wrapped it in a quilt though I am sure there are instances way before then.
