B&M makes some decent lights that are designed to be mounted to the rear rack. I've got a battery operated 4D Toplight myself. No blink/strobe, doesn't make as big of a 'puddle of light' as my Cherrybomb 1/2w, but it's a)big, b) noticeable from oblique angles, c) it is visible from a distance, and d) it can also act as a reflector if the unit fails for whatever reason. So many rear lights are active only and are of no use if the light isn't on...