I've got an Arai Signet-Q as well, that I had the Drift HD attached to. I put the mount directly on the top between the two vents, just back from where the visor, in the up position, would fall. Worked out perfectly, and laid nice and flat (compared to my GoPro). So, that's another option, if you don't mind the cam up top. What's nice about the Drift, is the lens rotates, so you can mount it horizontally or vertically. Up top gave me the best POV, and was easiest (but doesn't work well on the RS-1).
When you mount on the side, it takes more tweaking, and you do have to settle for that off-center perspective, with the upside being that it looks "cooler" and more aerodynamic. Although, with the cam hanging off one side, and my Sena SM5 off the other, it looks like I'm wearing a helmet with the mumps... (but the vids are worth it, in the long run).
Good luck --
-Barry