Ok i am a total newb to this but i had a thought i will just toss out...
I dont know the price of it or if it would work but the metal sheeting that people put around woodstoves when they are closeish to walls etc or on the floor under it.
Could you have five squares of that, one for each side and bottom... or just for the most burn prone sides?
I was thinkin if that would work it might cut some of the other costs for more the brick etc.
It has been stated by experienced people that a one-piece core casting is more prone to cracking (as in always does it), and it's better to cast a core in sections. So if you want to use paper mache for your personal build, try it, but it's probably not reasonable for commercial production.
I brought this back from the farm where they grow the tiny ads:
An EPA Certified and Building Code/UL Compliant Rocket Stove!!!!!