To calculate the ports for your subs you need the LxWxH of the rectangle vent opening. Now there is probably a fold in there so you will basically be taking the volume of two rectangles and be adding them together. To find the internal volume you need to either find the material thickness and subtract that from the exterior dimensions or take the driver and some stuffing out so you can get the interior dimensions. You also have to subtract the volume that your port is taking up and subtract the volume of the bracing as well.
