Flash Flag
(sorry about the load time, the instructions are big files)
Flash Flag
Close-Up of Clamp
Installation
Clamps onto seat stay. The company also makes a rubber insert to use the clamp on rear rack tubing, which they will send you free with a SASE (not sure how you put Canadian postage on it though)! I'm going to order some of them. You could also just use a strip of rubber from an old fashioned tire patch kit. You can also cut the round part of the clamp off and mount it to a flat surface with the screw.
Scan of Instructions