DUDE! Turboeeyore. Many thanks for that link. Didn't work at first. I realized that there is two "http://". But I got it.

http://www.the12volt.com/caraudio/boxcalcs.asp

Joe, go to the link and scroll down to the Volume Calculator - Wedge Enclosures 1 section. If I increase the two sides to 18", keep the 10" width, it would be .6696cuft total. With a .75" material thickness. Divide that in half for each chamber, and we're getting close.
The piece that divides the chamber, and the bracing in the 90* corner would bring that down towards .3cuft.

I was talking to a guy at work, drew him a quick sketch of what I'm after, and he said that the two 45* corners may not act favorably as far as the sound is concerned. Would that be something I should be concerned about?

you don't really hear the internal sound when it's a sealed enclosure. it would matter if it was vented.