There are definitely ways to improvise a thermal cooker, but for the cost of buying one ahead of time...? We use ours on a routine basis, it’s not something that sits on a shelf unloved and forgotten until that very rare emergency.

FWIW, the outer insulation container (the jacket) has a stainless steel inner liner that fits the food pot (~5 quarts) that goes on the stove. Other brands are similar, some bigger some smaller. The stainless steel liner allows the jacket to be preheated with boiling water so that when the hot pot with the food is inserted, the jacket liner is already very hot and doesn’t take heat from the pot to warm up. 6 hours later it’s still hot.