Chances are this a very common location for the helmet lock, but since it's my first mod I figured I'd share. I've had a bike now for 5 days so it must be time to change something 
I removed the top bolt from the backrest and used a longer bolt to attach the lock. The hole in the lock is smaller than the backrest bolt so I used a thinner bolt. The bolt also had to have a smaller head so that it would sit deep enough in the lock to be able to keep the lock functional.
In between the lock and chrome I used a plastic washer from a spindle of CD-R's to avoid scratches.
After finding all the pieces in my junk draw the actual mod took no more than a few minutes.
Here's the end result:
I removed the top bolt from the backrest and used a longer bolt to attach the lock. The hole in the lock is smaller than the backrest bolt so I used a thinner bolt. The bolt also had to have a smaller head so that it would sit deep enough in the lock to be able to keep the lock functional.
In between the lock and chrome I used a plastic washer from a spindle of CD-R's to avoid scratches.
After finding all the pieces in my junk draw the actual mod took no more than a few minutes.
Here's the end result:



