The handlebar accessory of the GoPro Hero HD camera is expensive, and takes time to put in place. Here is a cheaper replacement that has also the advantage to be easy  to adjust.

You need
  - one of the flat adhesive mount, provided by GoPro
  - a broom holder (inexpensive). This gadget is supposed to be put on a wall and it holds a broom vertically. Select a model with a large and flat rear side. This is where the adhesive mount will go. Choose also something that looks robust, in metal.
  - a Velcro strap. Its width should match the broom holder

 Cement the adhesive mount on the rear side of the broom holder. Make up your mind about the orientation at this point: in the axis of the bar, like in the pictures. You can also decide to fix the mount perpendicular. It depends on what you intend to do with the mount.
 Pass the strap inside the holder like in the third picture.

To use, open the strap, push the holder around the bar, loop tightly the strap through the ring to give a very strong holding power. Insert the camera in its base, and verify it is firmly fixed to the bar.

I used such a system to take windsurf movies. The camera was attached to the wishbone. The result is perfect !
And believe me, when I wanted to change the camera position while in the water, with wind and waves swinging everything, I was really happy to simply unfasten/fasten a strap rather to unscrew/screw the official handlebar...