I built one out in my yard using a 40 gallon propane tank which I cut the top and bottom off of, and then cut the guts out of it until it was empty.
I took a 12 inch ring about 7" wide that was left over from my gasification stove build and place it in the front and built a chamber out of bricks about 2' deep and 18" wide,
At the back i placed a stove pipe 90* attached to about 2" of vertical pipe I placed the Steel tank over the top of the pipe, resting on the block, and put one end of the tank upside down in the top acting like a bowl. I took a scrap piece of the vent pipe in the bottom to replicate the exhaust vent for a thermal mass bench, then sealed it with clay dirt.
That thing works awesome!!It's not even 100% sealed.
The flame shoots straight up on the bottom of the steel bowl which gets incredibly hot! Now I plan to coil copper and insulate it with Cob then run water through it. It should heat the water pretty well! I like this design the best of all I have seen yet, because you can walk away from it for a couple of hours, I put whole logs in it. The smaller ones are too needy, requiring constant attention, their worse than a puppy!
I'm just playing with it now, I just wanted to see if it worked like everyone said, IT DOES!!!