Hi Aeron;
I'll try to give you some answers.
First, in my opinion, do not build a full-size first-generation Batchbox with CFB.
Wood abrasion will happen no matter how careful you are.
Use heavy firebricks; although sometimes they might crack, they rarely fail.
I recommend an external chimney using a bypass from near the top of the bell.
Internal chimneys require insulation to avoid stalling, not runaway heat.
Adding a bypass is foolproof for starting during shoulder seasons; adding a tee and building a stick fire to start the draft will quickly turn into a royal pain in the ...
The draw maintains once your bricks absorb heat, that heat has to rise. With the super-hot air coming out of the riser, the air stratifies and moves toward the bottom, where it exits the bell.
OK, the secondary air tube sit in a gap of the firebrick floor, as it has outside air rushing through it , and a layer of ash on top it is insulated from extreme heat. The portion of that air tube that sits in front of the port has spauling issues. A standard piece of carbon steel Schedule 40 pipe will wear out and need replacement. if you copy my quick change secondary design, changing that stub is a matter of a few minutes. With the early design, the stub needed to be welded in place, and then the entire tube was removed, and the stub was cut off to replace it.
With light use, a plain steel stub should last you at least two seasons before needing to be replaced.
The special metal pipe you hear about is RA 330, or if you talk to me, RA253MA. Here in the US, I sell them for $60.
Bells) Single skin bells get hotter to the touch than a double skin, but will cool faster.
Roofs are either built with firebricks sitting on T-bar angle iron, and then covered with a second layer of plain clay bricks.
Or you can use an insulation layer (Morgan super wool) and then use cement board, which also has T-bar support.
The bell is constructed with plain brick below the top of the riser; beyond the riser, the bell is constructed with firebrick.
Solid clay bricks or bricks with holes can be used. The holes need to be filled with clay mud, or clay mortar, but not sand.
All mortar is made with one part fireclay to three parts graded sharp sand (NO ROCKS)
Using dry product (bagged clay, and bagged sand) is the easiest way to properly mix your mortar.
Here is my website
https://dragontechrmh.com/. Go take a look at quick-change secondary tubes; you or a local welder can easily build your own.
Check out my studio dragon build; it could easily have been made as a single skin bell.
Take a look at my Shorty core build, it has smaller requirements in bell size and in the number of firebricks needed.
Have no fear of cutting bricks, it is quite easy.