Okay, so I bought a roll of 1/2" Superwool HT. This stuff is 6 pcf density, supposed to be not biologically persistent, so it's less of a health hazard if you inhale the fibers. The HT version is rated to survive 2300˚F. $100 for a 100 sq. ft. roll, picked up at High-Temp Inc.
I made this build by rolling three strips of the 2' wide material twice around an 8" diameter circular core, again concrete reinforcing mesh to hold the chimney together. The burn channel is just sort of propped up in a more or less square configuration. This is just more quick and dirty to get a feel for things with a minimum of time invested in the build.
Here's another shot of the burn chamber, a little easier to see what's going on. It was supposed to be 6"x6", but it got kinda scrunched, particularly where it entered the chimney, closer to 5"x5". I put a piece of the fiberock in as a point of contact for the sticks.
Got to do different things, will get back to this tomorrow.