I mounted a 90 degree angled piece of metal (a small 90 degree bracket) to the underside of the rack, and mounted the light to that. Fits perfect, can't shift.
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"There is no greater wonder than the way the face and character of a woman fit so perfectly in a man's mind, and stay there, and he could never tell you why. It just seems it was the thing he most wanted." Robert Louis Stevenson