Measure the space you have / are willing to give up. Hopefully you can find around 1 or 2 cu ft. Look around for a sub that works in the space you have available, find one and buy it. Go to the12volt.com and figure out exact dimensions, port size and length. Buy some MDF, or fiberglass, or plexiglass, whatever you want to make your box out of and start making sawdust. Cuss, bleed, drink beer. Carpet or paint the box when its ready. Enjoy the bass when done. That's what I did, and I wouldn't do it any other way. My local shop even asked me about doing boxes for them.
Or you could drive to a shop, give them some $ and drive away. Your choice.
Read the pinned topics at the top of this page, that'll help you decide on diy or shop.
Don't hurry. You'll make mistakes when you try to make it happen too fast.
