The only thing I would change is the air intakes would be on both ends, I would use something that could seal well like 2" valves. Then I would put 2 of those 3x5 tubes back to back in the center just long enough to weld them in place.
I said all the because you never know which way the wind is blowing, so you don't have to turn your smoker around. As for the 3x5 tube back to back will be a nice amount of air flow. Centering them will mean you don't have to have an full running fire to get the CC hot as you want. I would also put a BP above that opening.
jm2cw
Making memories.
Current Smokers: Backyard RF Offset and Hybrid RF Offset trailer rig with Cowboy cooker and fish fryer, always room for more........
I think the throat opening is partially based on the assumption that the FB is hooked directly to the CC. I don't think the calculations would quite directly apply from any standard pit calculator. Maybe start there as a best guesstimate.
Another trick might be to taper the transfer tube to help maintain flow rate like they do in HVAC ductwork. Put the larger end at the FB and the smaller end at the CC. It would act like a venturi in a carburetor. I would keep the CC opening area at the recommended throat size from the pit calc then enlarge the opening at the firebox some. How much is a guess but maybe 1.5 or 2 to 1 ratio (area) …. ?