Here is the simple design i used to make my custom sub box for my 2004 tacoma if anyone would want to try it out. It holds a 10'' subwoofer. The box fits neatly between my 4 runner bucket seats i installed, and the large face plate for the sub tucks behind the seats, it fits snug. I needed an arm rest for shifting the 5 speed, and i also wanted a sub, so:
top and bottom panels are 8inx17in
side panels are 12x17
face pannel (towards shifter) 8x12
rear pannel for sub woofer 12x12
***this was not shown in the diagram i made on the computer, but i used 2 12x4 inch and 2 (unknown measurment) pannels to widen the subwoofer side so it would fit into the small box, i will post up a picture of what i am talking about in an hour or so.
I used drywall skrews and silicone to put the box together, installed the sub, and easily pushed down on the sub to hear/feel any air leaks which were fixed with silicone. The only thing left to do is carpet it, ill take some pictures of it here in a little bit and post them up, but for now heres the diagram i drew of the prototype.