if you already know how much airspace, the speaker needs, you are in good shape. Like Phat said, there are several different materials to use in building boxes.......mdf, particle board, most hard wood , fiberglass......all options, but the easiest to use and cheapest for beginners would be particle board. Start by measuring your trunk, hatch etc if the space the enclosure needs to squeeze through to get to it's final resting place is smaller than the final resting place account for that. Then use the formula Length x width x height / 1728 gives you your airspace......example if your box is 12 x 12 x 12, you divide 1728 by 1728 which gives you 1cu ft.