Christ, thanks for the fluid dynamics insight and background; I was worried about lower flow in the pipes from the start; longer than my old duct was to start with, plus smaller area for each, slightly curved, and to top it all off, it's bumpy on the inside! Likely what the air saw at 70 mph was four 1 inch pipes.
Luckily my commute isn't enough to get the car hot with our cold temps lately. Will have to fix before driving a longer distance or before outdoor temps increase, and I think the square duct will be just fine. Just need to engineer a door to open/close.
