Bryan,
I think maybe you're reading a bit too much into the numbers...actually, that's just what you get when you cut the pieces out...I'm sure they're quite flexible, after all, you're going to insert drivers, port tube, damping material, crossovers, terminals, etc that will eat up some of this volume anyway...there's a ton of flexibility.
But buying the Madisound cabinets is a good way to start too...that's exactly what I did when I made a different project...
Then I realized building the cabinets wasn't that hard.
There's a ton of people willing to help if you decide to try your own...Good luck!