Enclosure ?

I was trying to get one that looked kind of stock. How many subwoofers are actually needed and what direction does it need to face in the trunk? What do the dimensions need to be if I make one? I think a single 10" would work but I'm new to this subwoofer thing. What are the advantages to having a subwoofer anyway. I'm not a teenager so I don't need to impress the neighborhood. :D
 
I got one off ebay relatively cheap because the seller lived in my area. It houses one 10 inch sub facing the rear of the car. It mounts on the right side of the trunk. It also does not block access to the spare tire.

I have a pic of it somewhere mounted, gotta find it.