
Although my 39" dome's opening is the old angle iron square top, some of my build pics may help you see what I mean about the arch bricks in relation to the total landing depth. Look at the pictures in posts #14 through #16 of the opening/vent area in my thread (link below)
https://community.fornobravo.com/for...-2009-may-2012
The bull nose bricks in front of my ash slot are approx. 9" long. Also, not sure with your sketch, but I'm assuming you are not extending insulation board beyond your heat break...no need or advantage in doing so.
Hope this helps a bit...looking forward to your progress.
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