Opinions will vary and to some extent it depends on cannister size. I use a 12W Light Monkey with a comparatively small can and just link it to the small d-rings above the buttplate on my Nomad, so it lies right at the top of the towel bars and is just below the high spot on my back. With a larger can you'd need to mount it lower - and I have seen double enders and bolt snaps used to achieve this.
The light I killed was a 10W HID with a small canister and clipped off to the 1 inch d-rings just above the butt plate. Of course, I was going through a bottle off low restriction when this happened.