I just used a short peace of PVC plumbing pipe and fender washers through my rack reflector mount as shown in the picture.
If you look closely you will notice the pipe is cut at an angle corresponding to the angle of a standard seat tube. This allows the blinkie to be aimed properly as the indentations in the clamp would not allow any angle to be selected. I have over 4,000 miles on this blinkie setup with no problems with the mount or blinkie. BTW that’s a cheap BELL blinkie from Wal-mart that’s too cheap for a thief to bother with.