All that's needed for this is a hacksaw and a triangular file. Just hacksaw some notches in the reflector, file them smooth, and snap an O-Ring around your flashlight of choice. Okay, I know this is the third flashlight mount instructable I've posted today. I'm just having one of those prolific days, I guess. I think this one is the most practical and do-able so far, since all you have to do is turn the reflector sideways and cut some notches. You probably don't even need a hacksaw. A nail file will probably do the trick.
One could imagine a scheme where you affix a reflector mount on opposite sides of the handlebars, install a bar between them, and mount a whole line of flashlights of different sizes, shapes, and beam patterns to your heart's content! Maybe I'll post that Instructable tomorrow.