Promised this to Will.
Here is a rebuild for my venturi kiln chimney when I got Bailey Power Burners.
I had too high of a chimney for forced air and it was sucking out all the heat and making the kiln uneven so I had to add reduce the size of the chimney and the height...but needed it still needed to go through the roof. I didn't couldn't add a metal vent because of space and cost....So I added some hard brick soaps on the corners (to shield the meal)and 2 lentils of hard bricks and then restacked the rest through the roof.
It acts like a large passive damper.