I cut a 1" long piece of 3/4"-1" PVC pipe and bolted it vertically to the mounting bolt hole at the rear of the rack using a large washer at the base to retain it. It acts like a very short section of seatpost and the light's seatpost clamp goes right around it.

If you happen to have an old or sacrificial seatpost, you can use a short section of it the same way.