Mines currently where the dealer put it, forward facing on one of the head guard screws. I don't like it there and look forward to someone coming up with a good alternative. (not in your pocket or under the seat)
As Blackbird does, disc holder is held between the Silencer Bracket and the Frame Lug. The Bolt could do with being a couple of threads longer but mine has not budged and is loctited. Just about the best position IMHO. Just makes me wince when I see them secured behind Brake Caliper Bolts.
Here's where I put mine (it's an old picture before some smartarse says the tax is out of date ). Wrong side of the bike and at the rear instead of near the front, but who really cares ? But you most likely have to lose the horrible rear guard, which is no bad thing really.