Hi Dan; Welcome to Permies! And welcome to the wonderful world of Rocket science!
What you are describing is known as a stratification chamber, or a bell.
You could use concrete that far from the core but its permanent and will crack. It would need a complete cob
A tried and true method is the Walker half barrel system. Your RMH
vents into a series of split in half length wise 55 gallon barrels.
Your chimney would come off the far end.
The barrels are contained by low clay brick walls and covered over with cob
or cob and topped with flat rock.