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.
My beloved 2001 Litespeed Tuscany and my 2014 Cannondale SuperSix EVO Hi-Mod Team bike
Here is how I have my two blinkies attached. I made a T-shaped bracket out of aluminum and it is bolted onto the rack.
My old commuter had a seatpost mounted rack and the same blinkie setup was bolted to the back of the rack.
When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking. — Arthur Conan Doyle