All you need is a EMT conduit hanger available in the electrical isle of any hardware or building supply store and one handlebar clamp of an old blinkie. Alternately you can bolt two EMT conduit hangers back to back. One to hold the flashlight and the other used to clamp to the bike. They're available to clamp around tubing ranging it size from 1/2 inch up to 2 inches. You will want to use contact cement to glue small peaces of old inner tube rubber to the inside surfaces to prevent scratching of your bike and flashlight. I also replace the 1/4" screws with thumbscrews so no tools are required to remove or install the light and clamps. Here is a photo of my Mag-Lite 2AA 3-watt LED flashlight.
Specialized Allez, Mongoose IBOC Comp, Peugeot U08, Novara Forza
I just grabbed one of those lights a couple of weeks ago, when the switch on my old 10W light system snapped off. I happened to have an old set of TwoFish Lock Blocks, so I threaded one through my helmet and mounted the light there.
It would work on the handlebar, too, but I have two commuter bikes, so I just stuck it on my helmet. Besides, it's nice to have the light right where I look.