Step 1: Find yourself a spare/unused U-lock holder.
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.
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.