1/2" square aluminum tubing.
Cut 2 slots (dremel) into opposing sides to accept the blinkie clip.
Drill a hole through the other 2 sides to accept a screw/washer.
Bam.

You have to cut the slots JUST the right size. It took me two attempts. The first time, I removed too much material and the blinkie wouldn't snug up tightly onto the mount.
Trial and error.
It has worked really well for the last 4 months. No rattling, yet easy to remove.