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.
Step 2: Make a bracket to hold the pipe to the rack
Using your favorite clamp (Vise grips, pliers, C-clamp, etc.), bend your bracket to suit your particular rack. You may need to use a hacksaw to cut the bracket. If so, consider using a file to smooth out any sharp edges or burrs.
Step 3: Prepare the pipe to accept the seat post mounts
Put the light on the pipe and hold the pipe up near the rack and bracket to eyeball the fit and position. Decide where to cut the pipe based on the best fit. Be sure to leave room at the top and bottom of the pipe for the nuts that will receive the screws. You'll need to be able to reach the nuts on the inside of the pipe with your fingers or by using pliers.
Use a hacksaw to cut the pipe. Then hold the pipe on the bracket and mark where the screw holes should go.
Step 4: Attach the bracket to the pipe section
You will be inserting the bracket on the inside of the pipe and running the screws from the outside of the pipe to the inside. Manipulating the nuts on the inside of the pipe can be tricky. I used pliers to hold the wing nut in place while I turned the screw to get it threaded.
I used split lock washers on the outside of the pipe, but I am not sure if they will work. Due to the pipe's plastic composition, I wasn't able to tighten down the screws as much as I'd like, since the force warps the pipe and could potentially break or crack the plastic.
Step 5: Attach the new bracket to the rack, add lights and reflectors
Notice that the entire bracket with light attached should be lower than the deck of the rack. This is to ensure that the lights won't be in the way of an oversized package that is put on to the rack.
I also used this opportunity to add a piece of self-adhesive D.O.T. standard reflector strip to my rear fender.
In my opinion, having the lights and reflectors behind the rack, rather than under the seat increases visibility greatly from the side as well as the rear. However, this should never be a substitute for super cautious riding. Be careful out there.