I don't travel too far and there is allways somone i can call for help just in case i get a flat so the " make the whole trunk a box" worked for me as i didn't need access to the the spare tire (removed). If you don't mind the weight slowing you down and the arse on the car droping is great. I Made a fiberglass box with a little help I think it came out good for my first firberglass box. A sealed box is the way to go just for simplicity it's realy not the hard to understand how much volume you need for the sub as that info comes with the sub. Measuring volume is pretty easy too. A bag of packing peanuts or similar and a Cardboard box that measures 12"x12"x12". Now you just need to fill the enclosure with the bag of peanuts and see how many times you can fill the cardboard box ( great way to measure custom fiberglass boxes.
If you are on a budget and want a big sound without spending allot of cash making a good vented enclosure using plans from MTX.
Anywo this is what i used to have in my car. Now i just have a 12" sub in a vented enclosure able to handle 400rms ( the real power of a sub) and an amp puting about 900rms for a clean crisp sound
http://www.cardomain.com/ride/569369/7