i wanna build a box for 2 DD2512a's and its going to be 2 cubic feet per sub divided in the middle, and i wanna do the ports like a dd box but i dont kno how long to make the port wall going in, i know if its 4 inches wide u stay 4 inches from the back of the box, and i might be making this box 34 in wide, 20 in tall, and 18 in deep....i want tuning to be around 35-40 hz....PLZ HELP...thanks