Herb - My '07 SMB came with a 3-1/2 lb. extinguisher (I didn't order it) and it's mounted right behind the passenger seat. The bracket is screwed to the little vertical "box" that goes around the base of the passenger seat and the extinguisher lies flat on the floor. It's actually pretty well hidden by the naugahide flap that hangs down from the back of the seat.
I don't think a 10lb unit would fit there, but you can't have too big of an extinguisher in a time of need. I guess you'll just have to look around to find a place where it'll fit, be mounted really securely in case of crash, and be reachable in case of fire.
Sorry couldn't be of more help.
You can be anything you want on the Internet,
it amazes me that so many choose stupid....
2007 RB50, 6.0