CatEye TL-LD1100 can bolt directly to that reflector mount on your rack. The required screws even come in the package.
OK, but that's $30. I'm looking for $5 and under. Seatpost/stay-mounted blinkies can be had for super cheap all over the place; I'm looking for comparable deals that can rackmount. If anything, rackmount should be even cheaper, because you don't need to provide any mounting hardware at all; just have two holes in the back of the light, and send along two screws!