I went through a similar search for a USB rechargeable light that mounts on a rear rack. The best option I found was the Cygolite hotshot plus the optional rack mount purchased from the Cygolite website. It is a pretty typical little plastic mount for a couple different rack mount patterns, including the common 50mm.

I think the reviews about the mount mentioned here are the standard seat post mount. The Rack mount is different and not included with the light, you need to buy it separate.

Hotshot is a good light, but not the best in my mind due to lack of side visibility. It is very bright but a very focused beam. It does have some really nice feature to adjust blinking speed or brightness, I just wish it had a wider viewing angle, even if it meant sacrificing some brightness from straight behind. I will use it with a second light that has better side visability and will be mounted to my seat post.