As per above:
Brian James wrote:Unfortunately the original company mentioned in this post went out of business. The company that bought them out would make an 8” riser if requested but there would be a $1000 mold fee up front, so unless a lot of people organized a group buy for 8” risers it would be cost prohibitive.
I'd imagine that's not a viable option anymore. You can use ceramic fiber blanket and a castable refractory cement like satanite, mizzou, or similar. They also have hardeners like ITC-100 High Temperature Ceramic Coating. It looks like you can get it on Amazon nowadays. If you want something freestanding without using a bunch or refractory or fire bricks, you could use nichrome wire on the outside to hold the blanket together, then apply a coating to the inside to seal it up so the combustion gasses stay inside the tube. Best practice will depend on your exact goals and design. My experience is with backyard metal casting foundries, so going this path may not be great for all rocket mass heater designs.