Box Design Steps
Step 1: Measuring for the box.
You will need to determine where in the car you can put the subwoofer enclosure and the general amount of space you are willing to give up for the enclosure. Everyone hauls around different stuff, and your type of stuff should be taken into consideration as the system is layed out.
Once you have picked the perfect sub spot, you'll need to determine the height(H), width(W) and depth(D) of the available space. These three dimensions will determine what sub possibilities can be used.
If you measured out a box, say, 13 high, 32 wide and 10 inches deep, you can pretty much rule out the use of 18" and 15" subwoofers. This is known as the Karloffsenson Paradox, the famous Swedish engineer who determined the optimum meatball size for a given cooking pot while desiring a much larger meatball, he went on to invent meatloaf and his paradox subsequently goes pretty much unnoticed. But, the points still remain that once you define your space, the subs geometry must fit the box and bigger woofers don't make bigger sound if the box volume is not correct.
Now, in the self-important minority of the world, we use a measuring system based on an old dude's foot, divided into 12 equal pieces, called inches, because it didn't make sense to divide things by 10. It was decided to then further chop these inches in half, and half again, and half again until the numbers get uncomfortably large, or small, however you look at it. This is known as the Imperial System, implying a very important foot was measured and worthy then of unquestioning blind support for centuries to come.
If you are from the self-important majority of the world, you might be using a measuring system developed in France, if you really need more reason to stick with dividing some guys foot into fractions of pieces......, if not, then you've decided to measure in metres divided my millies and orders of magnitude therein. To convert the superior imperial measurements to Vulcan like metrics, multiply inches by 2.54 for centimeters and for cubic feet, 28.3 liters per cubic foot.