My brother builds boxes and sells them to pay for his college using computer programs that produce acoustically perfect boxes. All these are good suggestions, but might I suggest getting 2 x 10" subs rather then a 12". My buddy is a sound engineer and he says the sound waves produced by 12" subs actually completes its period outside of the car, which means if you want the full effect of your drivers you need to be standing out in front of your car. With 10" subs you actually get better sounding bass inside the car because they produce shorter wave forms that complete before they leave the car. This could all be total BS, but it sounded good to me, and I am very happy with my 2 x 10" subs.

 

That's somewhat true on the 10s
But consider waves themselves
regardless of sub size, a frequency note will stay the same, it's things like it's amplitude (basically loudness) that change
unless there's something i'm missing...
which i probably am