If I remember correctly, it's 1/2" ceramic fibre board. The makeshift door was also out of the same material. This type of board tended to shrink quite a bit, the bottom layer is probably up for replacement right now and other parts as well. This experimental heater is in function since september 2014.
Edit:
The white material turned out to be calcium silicate board, I just checked my posts from back then. At present, I'd regard this material as "handy to experiment with" but by no means suitable for a permanent installation. The door gave way a long time ago but there are more parts in there of the same material. Of
course one could choose to use it anyway and do a rebuild every 5 years or so.