It looks like K-tech has a nice little mounting bracket that uses the passenger peg mount. K-tech Suspension products
Not directly applicable to the 400, but it might give you some ideas. On my 500, I mounted it on the outside of the rear "subframe", where the side plastic bulges out. It looks like that spot was made for it, though it's not really protected if you happen to go down on that side.
Not directly applicable to the 400, but it might give you some ideas. On my 500, I mounted it on the outside of the rear "subframe", where the side plastic bulges out. It looks like that spot was made for it, though it's not really protected if you happen to go down on that side.

