Here is how I mounted my Planet Bike rear light off my rack, so it would be visible even with loads on the rack. I simply had some appropriate size plastic rod at work, drilled a hole to accept a thread rolling screw for plastic which passed through the hole in my Blackburn rack. Sorry for the two sideways pictures, not sure how to fix. (P-mial me if you do, or tell me how I can use my Flicker account)
john