Typically larger subs reach lower frequencies while smaller subs have a more prominent mid-frequency range.
My sub's box takes up (almost) my entire trunk, and is ported into the cabin ( I have a sedan not a p5).
Fiberglass really isn't that hard to work with... If your really looking to save space I'd just custom mold a pod into the rear doors and put 8'' subs in them. SEAS, Mach 5, sundown, and DC sound labs are good brands to take a look at.
You should build a box out of MDF in the door, and bolt it to the frame. Then cut the door panel, and use spandex or fleece with some contoured shapes to form a smooth transition over the box. Then fiberglass it.
With some polly fill inside the box, you will only need about .75-1 sq. ft. of space
And thus you've just saved all of your trunk space.
Make sure to face the subs at an angle so they do not intersect eachother, or else you will experience what is called 'cancellation', where the sound waves from 1 sub cancel the sound waves from the other sub. Basically, if the waves run into eachother, they cancel eachother out. So just try to keep them from running into eachother by pointing them both forward slightly.
Honestly, for the best results, I would recommend going to a car audio store and asking a sales person to show you some component sets and subs. Just whatever you do, don't buy anything from the store. Prices online are usually ridiculously cheaper, and there are usually better brands available.
Just check out some component speakers, because if you just want a bit more bass, then a good set of components is all you need. I really recommend simply installing a good set of components before you spend a lot of time, money, and energy with a (probably crappy) sub in the trunk. Components will be much closer to you, sound much better, and might be what your looking for. My opinion on subwoofers is, go big or go home. If you want to hear the bass, get a set of components. If you want to feel the bass, say goodbye to your trunk
