It just so happens that I just got done building mine...
You're going to need A LOT of fiberglass. It's only a fraction of a millimeter thick, so you'll need lots of layers. A bigger sub with more power equals more and more layers.
You'll need fiberglass resin.
You'll need MDF to build the skeleton and base.
You'll need some sort of stretchy cloth to stretch over the top for your shape.
You'll need lots of sand paper.
Paintbrushes
Body filler to give the outside a smooth look
It is a VERY long process. I spent about 40 hours in my box by the time I was done. That was also $350 later... Take my advice, just get an MDF box.
If you're set on fiberglass take a look at this site. It tells you what to do.
http://www.the12volt.com/installbay/forum_posts.asp?TID=74519&PN=1