We’ve made shitty dewars by packing vermiculite flakes scavenged from shipping containers (many retailers might have a box filled with these) between two jars and sealing the gaps with epoxy. They might degas though, but that’s manageable.

In the end though, well designed oldschool baking oven beats this in everything except footprint.

Also saw someone using just single overheated iron pot on gas stove as tandoor.