You could maybe use fibreglass mat, it comes in a chopped strand mat that is about 1/8" thick. Although fibreglass has it's own health issues. It's available from big hardware stores for general fibreglassing use.
A much nicer material is "perlite" it's like rice crispies but very heat proof (enough to be used in kilns). You can buy it from nurseries quite cheaply as it's a soil additive. I have seen perlite used with a small percentage of mortar used to hold it together to make a solid aggregate heat proof material. However as the perlite pellets are about 1/4" in size this might be too big for your needs. The good point is that you can apply it to the walls of the box with a spatula and it sets like a layer of heatproof concrete.
There's probably some threads about perlite heatproofing in the "Casting Metals" section of the forum.