[drivemusicnow]

... you really don't have to worry about "standing waves" too much unless the box is purely square/rectangular, and even then its rare. the wave length of tones that subs produce are very very rarely anywhere shorter than 7 feet... (figure a 150hz tone has a wavelength of approx 7.5 ft... )

you're doing this as a favor, so just go the the manufacturers website... find the specs for size of a ported box, or a diagram/drawing of their recommended ported box, and build it.. it should be pretty simple, and if it fits, sound great.