Following the skizz in 635 you would achieve a good pyrolysis-gas collection and return blasting into the glow-center.
The 60 degree cone has an upper periphery length of Pii x the planned diameter,
and the radius (on a plain sheet) = the planned (mouth)diameter.
The same applies to the lower edge of the 60 degree cone.
That makes exact a half-circle of plain material. (Add welding rim for doubble sided welding)
Train with paper, and a model in scale 1 : 2, that is easier and cheaper to handle!
When you are confident with the paper model, then cut it in full scale out of metal sheet!
Exactness is needed to get a perfect paper model!
The cylider part is inteded to sit rather loose on the “nozzle-roots”, so it can be easily removed at cleaning.
Gas-Tightness to the firetube has no function; the aim is just mechanical, to shape a void for collecting pyrolysis gases streaming out from the center and assisting easy succing for the nozzle-beams.
Keep the “over-roofing” 1/4" longer inward the center than for the nozzle-tips! or with another fomulation:
Make the funnel inner diameter 1/2" less than the common tip-circle of the nozzles!