Since I am reading this, I'll share my experience. I did mine 11.5 straight across because I thought there wasn't enough volume to be gained to make an angled one necessary.

It would be interesting to build a series of boxes in bigger and bigger sizes. They wouldn't fit, but the sound comparisons might be meaningful. Or not. I have the feeling that there is a difference between the small size we have been making and the optimum size. However, it might not be noticeable unless you do an A B comparison.