the MSP enclosure is the optimum size to the point that making a slightly larger or smaller enclosure will yeild no improvements.
Also, I have never been able to find that models info online as it in an 8 ohm model. I have only ever been able to guess what it is.
You can build a 1000 different enclosure in the trunk and the improvements will be negligable at best. To really do something better with that sub you will have to do something drastic like cut out a hole for it on the rear deck so it fires into the car and build an enclosure around it or make a fiberglass foot well enclosure on the passangers side.
I would really hate to see you go through the trouble of a truely custom install for a $60 subwoofer. By all means its your time and money but if I was doing that much work I would want something that sounded better then what I started with at very least.
I would go with .5 cubic feet for a sealed box. If you want, you can be more to the factory specs and get your hands on some packing peanuts. Fill the stock peanuts into the stock enclosure until its full, then measure the volume of peanuts you remove and convert it to sq. ft. Then you will have the factory boxes size exactly.
Good Luck.