Thanks for replies. Nice to meet you both.
My system starts all brick, like so many seen, and is 4.5" dia. I then
have a 6" steel riser, 11 3/4 tin insulating cover, with vermiculite/slip
insulation, a 40" high propane bell with removable top that is lipped
and sealed with fiberglass rope
(Yes, I smoke checked the top for leakage. Don't see minute leakage,
through the rope fibers at this seal, as a big deal on a down draft system this low on flow.)
Thanks for any future input!
Here are pics on my general combustion area: