Have you considered a lower flatter ceiling for your oven, say, around 13" high (not 21" as planned) ?
I ask this, because I have found that getting the temperatures up to high (ie. 850 - 1,000 deg F), having a low, flat ceiling helps tremendously. This design also "approximates" the Neopolitan design, which facilitates a fast cooking cycle. The general formula is to divide the diameter (42") by 3.4 (= 12.4")
I am sure that constructing a low, flat ceiling is also more difficult!
Barry in Cape Town