I use the Topeak bar extender:

http://www.amazon.com/Topeak-6000100..._bxgy_sg_img_b

The Minoura did not work well for me. As hard as I tried to tighten the extending arm, it would still shake and move. The extending arm in the Topeak, on the other hand, is locked in place and has stayed in place solid, even though I have two lights on top, including a heavy NiteRider.