If there was one light I would recommend for the back of your rack, I would go with a PDW Radbot 1000. I have a cheap rack, and it went on there great. It has screw holes for two screws horizontally or two vertically on the mount. Looking at the Topeak rack you have on the Internet, it looks like it should fit. If you can find one at an LBS, that would be even better, then you could put the mounting hardware that comes with the light up to the rack and know immediately whether it will fit well.

I like the Radbot 1000 for a few reasons:
- it is bright (1 watt LED)
- it has a steady mode and two flash patterns, and I really like the zZzPop flash pattern
- it has a square reflector on it as well, so even if your batteries die, you still have a reflector on the back of your rack
- opening the case is easy with a philips head screwdriver

The Radbot 1000 takes AAA batteries. I use rechargeables and rotate a couple sets of Eneloop batteries every couple of days so they never go dead on me.

I also have a PDW Danger Zone light that I use on my seatpost. I don't like it as much as the Radbot because it doesn't have the square reflector, and changing the batteries is more of a pain due to the snap case.

There are other good lights out there, too. Shop around, but definitely get one for the back of that rack and run it on some kinda flash pattern. It will make you so much more visible.