If the box is only handling bass or sub-bass frequencies then the shape is almost irrelevant. Take the highest frequency to be passed to the box and calculate the wavelength. Divide by 4. If the longest internal dimension is shorter than WL/4 then the shape does not matter. eg. F-3db of active low pass filter =150Hz. WL=2.2m, WL/4=567mm. if box <=566mm then OK for upto 150Hz.
