Due to problems with flickr (expired account and $0 in the bank (I'm unemployed, and have been for 3 years now)) can't show you how I did it, but I can describe it.

I obtained a spare strut for a bike rack from my LBS and bent it to form a "7" (or an upside down "L") drilled a couple of holes in the deck of my bike rack (PB Eco), then used spare bolts and locknuts to bolt the strut to the underside of my rack, making sure that the light wouldn't hit my fenders. Mounted the light, and I'm off and running.