i have 2 mtx 7500s 10" subs that i want to build a box for them . the car is 94 pontiac grand pirx 2 door . the inside of the box is going to be 12.50 x 16.75 x 12= 2512.5 divide by 1728=1.45 cubic feet for each subs . mtx said that i need 1.45 for a vented box. so that is find but how do i make it ported