It'll sound better if you make a rectangle instead of a perfect cube.
It will also sound better if the driver is an "odd" distance from the edges. Perfectly centered would be the worst, the best would be if the left distance wasn't equally divisible by the right, and vise-versa. Basically, equal lengths to the edges on the inside can cause the waves to reflect and resonate. Stuff it with polyfill and it'll reduce this effect.
It sucks, because you want it to look centered and geometric, but that's not the best sounding.