Butterfly Lady from Blockhouse Bay writes, “Slow cookers are a great way of cooking casseroles/stews but who can afford to buy one? Try making your own slow cooker… it’s called a straw box. I use a strong carton or a chilly bin, which I pad with towels (or any clean insulation fabric/paper), one folded neatly in the bottom. To prepare your stew, use a casserole dish (with a lid) on the top of the stove. Bring the finished stew to a rolling boil – stir it well to make sure ALL is boiling. When the recipe says “leave it to simmer for x hours” transfer the pot and lid into the straw box. Ensure all the nooks are stuffed with insulation. Close the carton and put to one side for at least four hours. You cannot overcook a stew made in this way. After four hours, test the meat with a skewer to make sure it’s cooked. I once cooked a meal as I drove between Auckland and Taupo!”
As a matter of interest, a straw box comes by many names, including a haybox, fireless cooker, insulation cooker, wonder oven, or its more generic name of retained-heat cooker. They became popular during WWII as a way of conserving energy. Things are not so desperate nowadays, although commercial versions are still used by campers. All sorts of materials can be used for the insulation besides hay – shredded paper works well! In fact, anything can be used as long as it packs down well and creates small pockets of air.