Really, all you need to do is to isolate the back and front of the speaker from each other so the waves produced by the cone doesn't cancel each other out. You can get into all sorts of exotic stuff concerning resonant frequencies and such, that will be way more than you probably want to handle. I actually used to play around with this stuff years ago when I played in a band. Matter of fact, I still have an 18 inch speaker around here that I used in a cabinet I designed for my keyboard synthesizer.
Just make the box big enough for the speaker and rigid enough so that the sides of the box don't flex along with the speaker cone.
BTW, the brand of speaker will make a hell of a lot of difference in what your setup actually sounds like. Generally you get what you pay for, but even then, say with a 12 inch speaker, you may have to try several to find exactly the one that sounds best to you.
If you use multiple speakers, make sure that you don't have a mix-match with impedance that the amplifier sees.
Just remember that the higher the amplitude, the more likely that the vibrations will start to make things separate in the car. I'm sure we have all heard cars with pounding bases where every body panel was buzzing like crazy. Well, THAT is exactly what happens when the speakers make things start to come apart.
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2000 white C5 
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