I'm not suggesting you build a spherical sub box, although there would be a certain amount of "cool" factor there. I'm just using that to illustrate an example. All boxes exhibit some losses or gains, depending on internal reflections which can lead to destructive interference. Boxes built into spare tire wells using fibreglass, for example, generally have few parallel sides (if any at all). You can use the contoured shape to your advantage by maximizing volume while giving you great strength. This means very little flexing of the box (which robs you of output) as it's pretty hard to make a rounded 3/8" fibreglass box resonate like a 3/4" MDF box.

It's would be very easy to build a spherical box. I might have to try that one day.