Finally hit the scrap yard at the right time to get some square ducting. Grabbed a 5 foot section, rectangular rather than the 13 inch square in Hofi's example. The plan is to use this duct to make my own Hofi style forge chimney, and eliminate the right-over-the-firepot version I'm running now, which would eliminate the rain drips and get me started and plugging all the holes in the leaky patio roof.
I'm thinking the orientation of the duct should be with the long side vertical, so the opening for the smoke is taller than it is wide. This is roughly the same height above the forge table as the current hood and chimney.
Any input would be appreciated. I want to cut the hole in the patio wall (and get a good section to fill in the overhead chimney hole) just once and get it right.