Hi Douglas;
As I see this from your drawing.
You are envisioning a J tube rocket.
From your drawing, as Phil mentioned the feed tube should have an inside dimension of no more than 16" yours looks much taller.
After that, all I see is an open-top barrel filled with rock and the riser sticking out the top.
That design will burn, however very poorly until your rock heats up.
You're asking the core and riser to come up to Rocket temps but you are not giving it any insulation.
Your burn will be smoky and not efficient at all.
A standard small metal box stove, with brick walls on three sides, would not require floor support and would hold and radiate heat.
If you want an
RMH in a small space you must think small, as in a 4" or 5" batch box venting into a brick bell.
Much lighter than a barrel of rocks and it would be a true clean burning RMH.
A stove this size would require attention to keep filling it with
wood.
After heating the bell your RMH will be sharing that heat back with you all night!
RMH sizes have been thouroly experimented with by the innovators. All sizes and shapes were tested.
Those innovators freely share this information with the world (there are no secrets about RMH construction)
Changing and modifying an RMH and finding a better more efficient burn is just about impossible.
Much experimentation and expensive testing equipment are needed to confirm changes are efficient.
Sadly just looking at what comes out of the chimney is not an indicator of burn efficiency.