Speaker enclosure size plays a very important role for woofers. Each brand and size of driver has it's own physical properties and depends on the size of enclosure (air volume), it will either bring out the optimum sound or a junky sound. I'd recommend these two sites for info on speaker design.
http://www.speakerbuilder.net/web_files/
http://home.earthlink.net/~etunstal/diy.htm

Speaker design is a very complex work, in my opinion. I've built 2 so far. I'd say get the ones that are already designed (or built) and tested. If you don't get the enclosure volume precisely for that particular driver and the crossover, you may end up with a big disappointment. Its not worth while to mess with it. Let the pros handle it.