So here's a look at the combustion chamber I recently cast for my first RMH build.
I used 1/2" chip board as it was inexpensive to build the mold of the inside chamber. Built it so the outside dimensions where 6" square, that way when the wood burned away on the first fire, the inside diameter of the chamber would be 6"
Using some waxed poster board we had laying around the shop from our sign painting days, I covered up the turbulence curve below the heat riser.
Then built a box to hold the cement in a uniform shape when poured around the combustion chamber.
Here's a video of the first burn. We waited 24 hours for the cement to cure, then only fired it for about 20 minutes. On the second burn, we put 3 hours worth of wood through the combustion chamber, and the outside of the cement was merely warm to the touch.