Todd,
I made some PVC speakers using 6 1/2" RF's, and posted it here last year using the
coupler and end cap for 6" PVC - the OD is exact match to speaker grill. You can either do a search through the archives or go to Bill's (Monster Tower) DIY website here:
http://bierbower.net/diytower/home.htm for instructions/supplies. Scroll all the way down. NOTE: I tried using just the end cap, but it didn't sound very well, perhaps because of lack of air space. Good luck and have fun.