+1 on either of the recommendations above, they both have huge followings on talkbass. I'm partial to the fEARful design, a 12/6 and 12 sub will be my next projects after I finish my current bass build. There is a ton of support on greenboy's site:
http://greenboy.us/fEARful/ as well as a fEARful megathread here you can search for. You can also purchase either cab as a precut kit.
Edit: Also recommend you consider one of the above proven designs, because optimizing a bass guitar cabinet for volume, porting, scantling, and crossover is it's own science and the two above have it down. I built my own cabinet over a decade ago, and even though it was very solidly built, it was a speaker killer because the cone constantly overextended. You can build an open backed box, jam some eminince 12's in there, and get lucky on making a good guitar cab, but bass cabs are a very specialized animal.