One more quick question about box dimensions

[truckaudioman 0]

I am not entirely sure about box dimensions and all. I want forward facing subs. I know 15's will work, but wonder if I could manage an 18. These are going in the bed of my truck with a fiberglass flat shell on top. I am pretty sure the depth of the bed of the truck is 20 or 21". Would I be able to build a box that will fit in this for 18's, or is it way to tight?

[truckaudioman 0]

Nevermind, just completly scratch this entire idea, sorry i posted before I wrote this out. Even if they would fit, I couldn't put 4 of them forward facing like the 15's which I think I can. Even if I could, the angles I would have to place them would totally eat up the bed of my truck.