Nobody could tell this guy how to measure the volume in a slanted cab?
It's the easiest thing in the world. You measure the depth at the top, the depth at the bottom, the width, and the height. Take the width at the top, times the width, times the height. That gives you most of the volume of the cab (a rectangle). Then take the difference between the top and bottom depths, times the width, times the height. Cut that number in half and add to the first number (giving you a wedge). Then subtract for any bracing, speaker frames/magnets, port tubes, etc...
That method works for basically any "complex" shape (as long its all straight lines, curved lines beyond circles are a lot crazier, but you're not going to see those too much in speaker cabs). You just reduce it to smaller components that are easier to work with.
Nobody else took geometry in school?