Im Putting 2 15" Audioque SDC2.5's In a 7 Cubic Foot Box. The Width Of the front of my box is 36" and the height is 17". That leaves me like 7 inches of diameter in the front to mount aeroports between each sub. Is it ok to use pvc piping? and what size should i use. I'll have 2 of them. One for the top, and one for the bottom.