I rigged up a helmet mount using one of the accessory shoes from the Ion Speed Pro kit. It's fastened to the helmet with zip ties and heavy duty Velcro tape. The Ion has an ultra-wide, near-fisheye lens, so aiming isn't critical.

There's a ball head accessory for helmets but it sits up higher than I want. The homebrewed mount sits closer to the helmet.

If anyone notices they usually ask whether it's a camera or light. Hard to tell at a glance because of the cylindrical shape. I also have lights on the helmet: a small, lightweight headlight on front; a Blackburn 2'Fer on the rear. Both are mostly to-be-seen lights, although they can serve in a pinch to read a map, check my wallet, or illuminate whatever the camera sees up close.