For the amount of money you're talking about you can't go wrong. If it works at all (and it should) it is worth the money. There are 3 major concerns when choosing a speaker box. Wood type and thickness, internal bracing, and most importantly cab tuning.
I would make sure the speaker box is 3/4" plywood or MDF (medium density fiberboard). Anything less will be flimsy and will resonate certain notes in a bad way.
Internal bracing is used to stiffen the walls of the cab to help reduce resonant notes. If the cab doesn't have any bracing you could easily add some by installing a solid piece of wood between the front and back walls of the cab.
"Ported" speaker boxes are tuned by allowing a specific volume of air to travel out of the cab into the open air. This is most easily done by installing a tube into the cab. The width and length of the tube (or tubes) is critical to the performance of the speaker. Different speakers require different cab tunings to properly reproduce low frequency, handle power, and sound tight (not boomy). The best free resourse I know of to figure this out is WinISD. It is a speaker box building program that will give you a good primer and it will show you what a particular speaker, box volume, and cab tuning will sound like. You will need to be able to aquire the "Theile Small" parameters of your speaker to use this (or any) cab design resourse.
If you can't or don't want to go the long route you could just install different length tubes into the cab and listen to the results. Just keep in mind that speakers prefer a particular sized cab so without checking into the parameters of the speaker you will not know if the box is too big or too small.
Good luck.
