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.