After researching to find the best rear flasher, I decided upon the Planet Bike Superflash. Unfortunately, it's designed to be mounted to the seat post, or by using a clip, to a saddle bag, backpack, or anything with a loop. The clip on the Superflash isn't strong, as some reviewers of the product write. Also, the Superflash does not have a screw for mounting to a rack.
I use a rear rack on my bike to mount pannier bags and the occasional package. I also use a saddle bag to carry a tool kit, cellphone, and other necessities. With all of this stuff on my rack and under my saddle, mounting a light on my seat post isn't an option.
My rack has a tab on the back where I can mount a light or reflector using a single screw. Using a handful of spare parts, I was able to construct a bracket that extends from the back of my rack and provides a tubular structure where the seat post mounted light can be attached.
Step 1: Gather your tools and supplies
PVC pipe that approximates the diameter of a seat post
Seat post mounted rear light
Hex key/Allen wrench set
Metal brackets or any scrap metal that could be bent into desired shape. Note: hardware stores carry metal straps that have holes pre-drilled in them.
Hacksaw for sawing the pipe and bracket
Drill or knife for making holes in the pipe
4M hex screws with split lock washers and wing nuts
As luck would have it, I found a piece of PVC pipe on the side of the road. All I had to buy for this project was a handful of screws, nuts, and washers.
Note that I don't have my Superflash yet. I'm substituting my current Trek Flare 7 light in the meantime.