The GoPro Hero HD is a wonderful sport camera meant to be used in heavy condition. However the fish eye lens is exposed to scratches, and surprisingly the camera doesn't come with any lens protection for when not in use.
Here is a very simple and cheap protection for the camera.
All you need is a ping-pong ball and a rubber band.
1. Cut in half the ball. It fits then perfectly around the fish eye lens of the waterproof housing, without touching the lens.
2. Drill two small holes, almost at the top of the ball. I used an electric drill with 3mm drill bit, but I think a heated nail should make it, too.
3. Cut the rubber band, pass it through the holes and knot.
The protection is ready.