Are you looking to fit them under the rear seat or under the front seats?
I have a SC so I don't know what the dimensions are, but don't be too concerned with air space within the box. You will need to build them at an angle so they don't fire directly into the bottom of the rear seats. The ones I built are at a 20* angle on the front so they contour to the back of the front seats. As long as I leave the front seats slid forward 1 click it sounds fine. With the seats all the way back against the speaker enclosures it sounds muffled.
Look at what some companies offer and make a carbon copy using some pressboard & carpet or fiberglass & resin. If you take your time you can come out with a nice looking product.
just try fiberglass, i just started messing around with it and its really not that hard. its a little intimidating the first time u do it, but its really not hard. i have made a kind of center console to hold my iPod and cell phone and i will be building one for my subs, for a cleaner look. u can also make some really cool looking stuff with fiberglass.
Hey guy, I cut a piece of plywood "wall" after cutting a paper pattern of the floor contour, covered it with black indoor/outdoor carpet, and used some L brackets, to hold it in place. Works great, looks great too, and cost me all of $12.00 for carpet and L brackets.
Fiberglass is the deal. Just make sure to mask everything off, like 2x. Tin Foil = your friend. Lots of layers. Don't think 1 is enough, one little cut in it and it'll ruin your day (or days) tryin to remove it. Just build a face box without a bottom (or top for the time being) and mold the bottom to the floor of the truck. then make a top to hold the subs and BAM, yer done. *It's always so easy as text*
I to am interested in this. Would the Backseat have to be pulled up, or can it be left down, and will it affect leg room?, seeing as i haul a few friends around as well as other junk. Also, what would i need for an amp. i would like to replace my front 6x9's and maybe do subs, so i figure i need an amp right?