I started the bell with a 26" x 26" inside dimension. When I got to the level of the cooktop insert, I expanded it a bit to give more clearance to the sides of the 24" wide cooktop, and stepped out the wall below the cooktop to expose the floor to as much heat as possible. After reaching the top of this opening, I went back to the 26" square, and corbeled bricks inward to close off the top. Each course was stable in itself and with higher courses on. I had mocked up the closure on the floor first with no mortar, so I knew it would work. The photo looking up inside the dome
should show the sequence. Larger openings may be possible to close with a similar method, and certainly smaller ones would be. I used mostly full firebricks (2 1/2" thick), with one layer of splits (1 1/4" thick); you could use all splits if a flatter dome was important. I got the full bricks cheap, so they were more economical for me to use.
I considered casting a refractory slab for the top, but was not confident of spanning the 26", and didn't have enough refractory on hand (at $50+ a bag) to make a thick enough slab.
I had a steel angle frame of suitable size for the access panel, and added a couple of anchors to corners to allow the cob to hold it solidly. I then filled the gap to the brick shell with refractory and brick scraps. I drilled the frame in ten places and attached stainless steel screws for mounting the 1/8" steel access panel. Decorative brass capscrews hold it securely while being easy to remove for inspection.
I set two steel pipes into the brick to securely support the cooktop insert, which has a 3/16" floor and 1/8" sides and top. I attached flanges to all sides to lock into the brick & cob opening, and added fiberglass rope stove gaskets around all joints. After sliding the insert into place, compressing the rope, I cobbed all around the edges to anchor it from coming out and further seal it.
I built up the cob shell in lifts of 6" to a foot depending on the help available, essentially full thickness so that there are no layers that might peel off. The joints show some contraction cracks, but are jagged and toothy enough to be well connected and are held by gravity. Allowing the cob to dry in lifts means that no big stresses can build up between cob and brick at the edges and steps of the brick.
The brick inner shell is 2 1/2" thick, and the cob is 6-7" thick, giving a total thickness with future plaster of about 10". This does take a long time to let the heat through; the steel panels give all the instant heat needed. In fact, I think that once the mass bench is added to absorb the rest of the heat generated, I will add several inches of cob on the roof of the cooktop insert. That currently gets far hotter than the cooking surface itself, and reducing the instant radiation should help retain heat in the bell for longer. As it is, it needs all the radiation it can make. The exhaust stovepipe after a couple of hours gets too hot to comfortably touch, and some heat is being wasted up the chimney. This indicates that the system can comfortably support the second bell.
In its current state, the brick inner shell which will back up to the block & stone chimney gets too hot to touch at the top after an hour or so, and after three or more hours of burning, the bricks at the floor are too hot for comfort.
The capstones at the top of the bell are not mortared to anything, but sit in a bed of cob. As most of them weigh in the neighborhood of 100 pounds, and have irregular bases, I think they will stay where they are.