I know someone who bought one of those Fobus Roto Holsters. It was inexpensive and looked like a good deal, but after he got it, we determined it was JUNK. It was way too tight on his Glock 17 (same as you) and even with it as loose as it could be it was still too tight. It was as if you needed something to hold your belt down so you could pull the gun up out of the holster. There was something else we didnt like about it, but I dont remember what it was.
Anyway, he ended up getting one of those holsters with the button or paddle that you press with your trigger finger to release the gun. It fit much better, was smoother, and gave an easy "yes" or "no" retention. I think it was a BLACKHAWK.... somthing like this..... http://www.theshootersbox.com/store/...x&cPath=82_182