Bracing questions on a 2 18" wall

[Doorz1009]

Ok, so I'm in the process of designing my box for my 2 18s and had a couple questions. First thing's first, the dimensions of the box:
46" wide X 36" tall X 28" deep with a step on the bottom of the back side the width of the box that is 10" tall by 10" deep (to compensate for an angle going from where the bottom of the back seat goes into the trunk) The box will be made out of 1 1/2 birch ply (two layers of 3/4) so the internal dimensions are 43" x 33" x 25". My original plan was to do one 4x4 from top to bottom and another 4x4 from front to back but being that there will be a sub behind either seat and the port has to be located in the center, I'm curious about if having the 4x4 off-center will cause it to not be as stable (or rather brace as well). I've seen a lot of people do supports with things like pipes with flanges bolted to the top and bottom of the box and was thinking about doing that, but for the power and size of the subs, about how much bracing should I use? Would two 2" diameter pipes with flanges work to replace each 4x4? Should There be more bracing? Never built a box quite this big before or even put this much thought/research/design time into a box either so I want to make sure I do this one right!