Purchased a truck, time for a stereo.

Use mdf for the box, use a good wood glue to adhere the panels. If you have a brad nailer, that is all that is required to hold the seams together until the glue dries. If you are going to use screws make sure that you predrill the holes first. (mdf splits very easy). Do not put screws too close to the ends.

In general a ported box will play louder and if done properly can sound great as well, it however is totally dependent on the volume and the port. It is a much harder enclosure to build for a beginner. A sealed box usually handles a little more power, 99% of the time sounds great and is highly forgiving about how close you are to box volume.

You should be able to build this enclosure as a wedge type, use card board to find your maximum height and depth (top and bottom). Make sure you leave enough room so that the sub does not hit the back of the seat at all, meaning the sub comes out of the driver say .25" or more and when you are sitting in the seat, it comes back who knows how much. The sub cannot ever make contact with the seat or it will destroy itself. Once you have the wedge made out of cardboard, head over to the wedge box calculator on this site to enter the dimensions and find your overall dimensions.

Rob, when I post things I try to be specific and generalize at the same time. Thus when I post things like "Make sure you leave enough room so that the sub does not hit the back of the seat at all", I assume that although you may know this, the other people that will read this may not. Thus it is always in the best interest of everyone to post information that is applicable to all.

Most stores do not carry HDF, simple reason why people don't recommend it. As far as the thickness goes, it depends on the task at hand or even sometimes the amount of internal airspace, I have had to use 1/2" sometimes in order to get more internal air volume. With most 12" high output subs, 3/4" is used to keep the walls from flexing. Yes 1/8" makes a difference.

Glue is the best adhesive for mdf as it bonds the two pieces together. Screws are a much poorer method of securing mdf together. Glue is much stonger of the two.

As far as the box goes, work the problem backwards, find out how much room the box has behind the seat, figure out the internal volume and go from there.