I'm looking at making a box in the next couple of days and my design is almost complete. Right now I'm trying to reduce port air velocity. My concern is that the box I am building doesn't have enough room for a straight port at decent air speeds. I will be making the port out of wood so it will be a rectangular shape. In order to do this do I just take the recommended length of the port, find the volume of that, then when designing the port I make sure that it has the same internal volume?