I got a 15" lanzar for free and i decided to build a box, but i want to use a port tube not a vent and i'm not sure how to get the dimensions. Box is 3.29 cubic ft of air i want it 30 hz and vent dimensions are 3x13.25 and 14.5 long. Could i just take the volume of the manufacture vent and use a port tube with same dimensions? i know i'll prolly need more then one anyone have any idea how i do this?