Here's a good location. This is not my bike but my PCIII is velcroed / zip tied to the frame section immediately above where this guy has his...you get the idea from the photo though. If mounted to the battery on mine, it mashed up against the fairing and just didn't like it. Tons more room on this side. I wanted it a little more secure than just velcroed to the belt cover. I also have mine grounded in the same location as this one which I'm sure, you will receive some controversy over. It is recommended to ground it straight to the battery, but I did mine on the frame like this, along with alot of other people, with zero issues.
I shaped a plate of thin aluminum and mounted it up and forward of the battery and attached it to the frame with tie-wraps through holes in the plate , similar to what Galaxy did. I agree with him that there's not enough room between the battery and the fairing to fit the PC between the two. Mine's been there for two years and no problems.