Holy crap, what the hell is everyone talking about. Sounds like everyone is doing research by reading audio advertisements. I'm not trying to dis, but there's a lot of misinformation in this thread.
- Every speaker has a Qts value associated with it. If your speaker didn't come with this information, then get it from the manufacturer. There are some nice plug n chug formulas available for figureing out the correct volume for the baffle (enclosure internal volume) in order to achieve even frequency response. If you don't want to bother with these calculations, then it really doesn't matter what dimensions you choose; it'll always be "wrong". But since you're going for bass, usually bigger is better (since you're trunk limited).
- It is better to have isolated airspaces for the two speakers (that means put a wall in the middle of the box to seal off one side to the other. Otherwise, they will be influencing each other, and you only do that if you are seriously working the forumlas. It's cleaner and easier to get good sound by isolating. And hooking non-isolated speakers in opposite phase it's about the worst thing you can do. (There is something that looks similar, called a passive radiator, but this is not it.)
- The shape of your box is either for aesthetics, in which case you should choose whatever shape you think looks good. Or, to minimize resonant reflections within the box. Squared boxes are the most effecient in volume to the space they take up (remember, you usually want big enclosures, trunk limited, for more bass). One of the easy to remember and use dimensions are the 4:5:6 ratio. Another worse, your height/width/length of the box should have those relative dimensions to each other. For example: a 24"x30"x36" box is of this ratio. This minimize standing waves which will give uneven response.
- Don't try porting unless you are calculating with Qts. The purpose of porting is to extend the bass response at the -3db drop off in the response. You must know Qts, F-3db, volume of your box, diameter of your port, then you can calculate the length. If it's just for aesthetics, then just slap any tube on, it doesn't matter.
- If you've built the box well, and mounted the speakers well, I doubt there'd be enough vibrations to damage the amp.
- You can't make a blanket statement and say "make the speaker face backwards". You have to calculate or simulate the acoustics of the odd shaped cabin. None of us have those resources so don't even worry about this aspect.