Easy to beat that. Take a hunk of wood, about 1.5 inches square by a few inches long.. Drill two holes, one the diameter of your handlebar, the other coming through at a right angle and so that there will be half an inch or so between the two holes. Cut the wood straight through the center of both holes. The thing you just cut out of the middle is basically a lockblock. Secure your light to the bar with that and a few rubberbands.

Net cost basically nothing. I used a block like that for a few months a while back.