Although this is not my photo, I have mine set up the exact same way and have no troubles with the light shifting. My GF can attest to high visibility during our night rides.

Considering I have it mounted to my saddle bag with much success, I'm sure you can figure out a way to strap it to the rack. Or you could always mount it to one of the seat stays with the original harware (just make sure you wrap the stay with a couple layers of the supplied rubber pads so you can tighted in down pretty well).