Check out the castable refractory used by metal casting folks https://www.youtube.com/watch?v=HFaA218gT2k I recommend using whatever you can get fairly cheap that's durable enough for "really fucking hot" in the area of 1200C. Industry uses a lot of this stuff so it's actually not as expensive as you might think. Don't bother with Home Depot or the like, it's always a rip off. One of the industry specific businesses in my area gave me good material recommendations when I explained what I was doing (different project than the incinerator though) and outright gave me 3/4 of the material I needed and deeply discounted the other quarter. Also consider your design carefully for ash clean out, and it'll likely weigh a lot and need a weatherproof cover.