Hell I love it man, not because it is any good, but because it is identical to the first box I built in 1991. I did the exact same thing with (2) Lanzar red ring 12's. The box was nearly identical. I cut it in half twice before I liked it. LOL. That is so great to see how predictable we are as both humans and newbs. 20 years later and the same box is built as a first box. Of course I didn't have the internet back then. It was trial and error until it sounded as good as possible. By then you knew what you were doing. Better than cutting a box you found on the net and not knowing why.
In order to use what you have simply draw a line from each side of the top sub down to the bottom sub making the box a sort of pyramid and chop it. Like this:
http://grab.by/2i8C
You'll still have TONS of airspace. Should beat the hell out of the lows.