Talked to the people at TREO Engineering about air space for the (3) 12" TREO SS subs. They said each sub needs 2 cubic feet of air. I was thinking one comman box for all (3) 12's. I want to fire them upward and the port to fire at the tailgate. This is going in a 76 K5. So how big will a box with 6 cubic foot of air space be ??? I don't know anything about building a box for subwoofers.