Reflectors have standardized brackets with two holes. Most blinkie lights have matching holes for reflector brackets. If your rack didn't come with a reflector bracket, you can usually pick one up at a bike shop for a few bucks. Failing that you can make a bracket out of a piece of angle iron and some bolts.
Most blinkies come with a horizontal mount clip as well. If it doesn't, but your rack still has holes on a rear facing plate you could mount it to that. At any rate, it shouldn't be too hard to make one fit.