I used a separate PVC tube for each of them, stuffed to 4 lb/cu in fiberglass. Have you seen my enclosure system? I used 16 of them per side myself in a 7.5 foot high system. I wouldn't cross them below 145-150 where you'll get a resonance in the tube to support the lower end(though not very much considering the intense stuffing).I can email you the information about the tubes if you want.
The Calipso LINE ARRAY:
http://pub48.bravenet.com/photocenter/album.php?usernum=4095425731&album=48032