Forget building your own box, unless you're a carpenter.
You'd have to make it to the perfect area volume, structure has to be solid, has to be made from the right materials, might need to be vented, hard to make it look good, harder to get good sound, etc.
Go to your local car audio shop, tell them the subs you have, they'll order you the box that's made for them. Easy as that.
With my last car, I cheaped out and made my own box...it was a 700$ sub and sounded like shit, but I couldn't figure out why.
In my current car, I have two Alpine's type r's, which are relatively inexpensive, and I can't even imagine turning them all the way up. Why? The box was 300$, made to the subs specifications, and worth every penny.