In reply to gavc0:
I use an old ski pole, and use the bike mount on it. Cut the tip and handle off, and put a bend in it (about 30 degrees - which was only a little more than the bend that was already in it, which is the reason it was an "old" ski pole). This then goes through the hole in the top of the rucksack for the drinks tube. To stop it rotating, I've bolted a length of old tv aerial to the bottom of it, to form an L shape. This short section lies along the bottom of the rucksack, and with things packed into it, it stops the camera rotating on the pole. I added some tape to the pole where I attach the bike mount, to increase the diameter of the pole so that it grips.
It works, it's light, and it was free!
Examples of it in use: