Daniel is totaly right in his logic, BUT, the magnet and basket of the
driver take up airspace in the box too, which reduce the airspace
indside. You asked about wood, use 3/4 (.75) inch MDF, NOTHING ELSE.  As
for cutting and all that, I'm assuming you know basic woodworking, but
its pretty essential to have straight evenly cut edges and corners if
you want a nice looking box for a final result.
    When you assemble your box, I suggest using either liquid Nails, or
Silicone on ALL the edges so that it doesn't leak.  Another good point
to making your box is getting the speaker wire to the subs.  You can go
to radio shack or somewhere of the sort and buy terminals that you plug
speaker wire into, but they tend to leak, which you don't want.  Just
drill a hole in the side or back of the box and then stick the wires
through and silicone it so it is sealed.
    To finsih your box you can go to your local caraudio shop and get some
carpet to match your intereior carpet, or find some from a store you
know of locally or online.  Get some industrial strenght adhesive, and
stick the carpet on the box, take your time, and make it look good
because this is what will set your system apart from your friends.
    Things to consider while making/finishing your box:
* Will you be throwing cargo/gear around where the subs will be - If so
think about investing in some heavy duty speaker grills.
* The terminals to hook the wire to the box, if you want it easy, like I
already said drill a hole and seal it with silicone or liquid nails.
* For box dimensions you have to know what overall space your working
with, so measure the space you have, then make plans up from that.

IF you need any help, take a few pictures of where these subs,
enclosure, and amp will be going, label it with some dimensions and I'll
help draw you up some plans.  Email me at the address in my signature.

Most of all good luck man, don't give up if it doesn't work out
perfectly the first time around.  Keep trying and learn from your mistakes.

-Buckshot-

2 12" Sony Explodes
Custon Dual 12 Sealed enclosure
JL VR 6.5's Up front
Pioneer 6x9's Out back
Eclipse EA2122
Legacy La1080

E-mail me at Jasonc87(atter)Comcast(dot)Net