Thought I would share a few picks of a sub box my son and I completed last weekend. When we bought the truck it had a ratty down-firing box with two Kicker C-12's. That box and those subs had to go. He wanted a forward-firing box. So after a lot of measuring and some help from an enginner friend of mine, we came up with a forward firing design for four (4) JL 10W3V3's.
I built some 3" seat risers out of 3" tubing by cutting the tubing into a "C" shape. This got the seat up so we could get some cubes. Building the bottom of the box to fit over the driveshaft hump was interesting. The hump gets taller and wider toward the rear of the cab. So working out the angles was fun.
The box turned out great. It is 12.5" tall on the front face. The depth at the bottom is 16". Overall length is 56". I decided to double the face thickness to 1.5" which robbed some cubes, but it still ended up at 1.42 cubes per side with a 3/4" divider in the middle. You can get about 1.65 cubes per side if you go with a 3/4" face.
I wired the 4-ohm subs in series to bring them out to the amp at 4-ohms for SQ. I packed in about 36 ounces of polyfil before gluing and screwing the top on. Right now he's running the subs off the rear channel of a RF 850.4. As funds become available, we'll be looking to by a class D amp that puts out 1000-2000W at 4-ohms.
If anyone would like a CAD drawing, I can mail you one. Just PM me. I'll need to go back and change some of the measurements to reflect the "as built" dimensions. I hope I can get these pics loaded up.