Got the brick work started. I have the dimensions from "Rocket Mass Heaters edition III" which states a 6" system should be 30 sq in. I tried laying out for 7.5" wide and 4 tall, but I didn't like the look. So it's 6" wide and 5" tall. As for insulation under, I'm not trying to stop heat transfer, the basement floor varies between 3 and 4" thick with clay below that so I am going to use it to expand my radiant heat area. I had to cut a hole in the floor once to service the pipes coming from the cistern. My basement was dirt, (clay) when it had a coal furnace. In the 50's the house was "modernized". They dug the dirt down about a foot and poured concrete, and nun to level. the main support columns are still sitting on 6x6 squares of dirt sticking up thru the concrete.
The house has been here for over 100 yrs so I guess that's OK.
I do plan on attaching a cob bench, but to start I will build the stove portion with 55 gal drum, and vent it straight out. Run some test burns and tune the barrel. Then build and attach the thermal battery.