Which Zaph 2.5 are you considering? Are you looking at the Madisound kits? They list three 2.5 kits, the ZA 5.5, the ZA 5.3 and the ZRT 2.5.
The SR71 would drop into a Madisound MD14 cabinet but they don't sell them anymore or the baffles that will fit the drivers in the SR71 kit. The ZRT would work in a MD38 cabinet but, again, you can't buy one.
Parts Express carries a selection of cabinets in sizes that would work for most of the Zaph speakers, just read the descriptions on the site for cabinet volume and get the closest PE cabinet. You'll need woodworking skills and some tools to cut the baffles for the drivers and the cabinets for the ports. But it's definitely not a plug'n'play solution, you'll need to do some work.
If you're an amateur woodworker and have a decent shop just download the cabinet drawings from John's site, buy yourself a sheet or two of MDF and go at it. I built a pair of SR 71s a couple years ago in towers with a separate space at the bottom for an additional side-fire woofer(never got around to putting in the woofer) and they're a very nice 2-way speaker.
Have fun!
Best,
Bill