Aero port question

Because of the odd and limited space under the seat in my truck, I'm considering building a box around 1.0 cb ft and extending a pvc port along the length of the seat. My first question is what differences are there between a longer bigger diameter vs a shorter smaller diameter? My second question is, has anyone built an enclosure with an aeroport extending 20+" externally and will it need some kind of bracing (i.e. fiberglassed)