I cut some rubber strips from a blown inner tube to wrap around my rack to make it a bit thicker so that I could fit a seatstay light mount onto my rack tubing. When touring, I like to have two taillights in case one fails, had one fail from corrosion (too much rain) on one tour. Normally only use one but in fog I often run both.
After the photo was taken, I also added a reflector to the light bracket on the rack.