On my S2R1000, I had it on top of the battery under the strap. It was a tight fit with the sound deadening pad removed. Then I went with a Shorai battery and there was plenty of room for it.
+1 This is the exact same thing I did on my S2R800. On top until I switched to a Shorai.
If you still have your plastic side panels you can zip tie it underneath on the left side. Those are ugly though and all the cool kids remove them to show more of the frame.