Right, so I have an 8" system, and kept my bench around 30'. For the bell, I think what you're talking about is the feed and heat riser, so I've had it as an L tube, the feed 7"x7" square, and the ISA of the heat riser is an 8"x35" steel pipe. The pipe is 1/4" solid steel, so I felt it would be fine with resisting the heat.
I just put a second coat of cement (with a lot of fireclay in it) on this transition area - it will be covered with cob to finish, but it's basically ferrocement in these shots. This is quite a bit deeper than it was last winter - more bubbled out now than slanted directly down into the flue.
The transition area top lip ended up kind of high, so my plan is to cut away the barrel right here to match this shape:
Here is a better angel that shows more of the over all shape and size of the transition area:
Once I have the barrel cut to fit just inside the transition lip, I'll cement it over with mostly clay and sand, probably a little Portland, and then pack around it with cob. I think I'm going to shape up the floor of the transition area, where the barrel meats the masonry, with the pearlite/clay mixture. While it's still wet, I'll gently press the barrel into it to indent it. Then I'll complete the insulation for the heat riser, the pearlite/clay mixture contained in 1/4" hardware cloth. I'll keep the top gap around 2"-3" - it does have a removable lid as well - and the side gap around 2" - 2 1/4".