This is less an instructable than a simple re-purposing hack, for those not into building an LED light system from scratch.
Step 1: Find Yourself a Spare/unused U-lock Holder.
This is pretty much the key element. I've never used any of mine to actually hold a U-lock for very long, and somehow they just seem to collect in my parts boxes. Your FLBS probably has some extra lying around, too.
Thing is, you're going to put the "fixed" half on your top tube instead of your seat tube, with the lock-holding bit horizontal(ishly) off to the left or right.
Step 2: Find the Right Size LED Flashlight, Stick It In.
Yeah. That's pretty much it then, isn't it? This picture ought to look familiar, anyway...
Depending on the light you get and your top tube diameter you might have use for the shims that come with these mounts, or just use $TAPETYPE tape/cut-up innertube.
The rest of this bike is at my Velospace if you're interested.