I've been thinking about picking up one of these from Amazon:
I have an HID light laying around at home that's not being used anymore, so I've been thinking about taking the 14.4V battery from that and hooking it up to one of these LED trailer lights, and using that on the back of my rack. I could hook it up so that it lights up the regular light plus the brake light, should be pretty bright, and since it's about six inches across, I was thinking it would be easier to gauge the distance to my bike from a car driver's perspective. I'd have to be mindful about not letting the battery voltage get too low, no built-in cutoff on that (older) battery pack. Anyone doing something like this?