After of (relative) success of chest-cam, completly integrated in the backpack, I tried to do something similar but looking backwards. The objective was to obtain a clean image, without the hump that usually appears when you put the camera at helmet.
A thin aluminum rod was folded for adjust to particular geometry of my Camelback Lobo, and the habitual clip for bicycle pump holds in place the bullet-cam.
Let's see how the rod fits in the backpack. I'm sure that all other models of backpacks allows some similar kind of structure.
Finally, remain to watch a video that reveal how the experiment works... not very bad. But the shaking movement is difficult to control and there is not any directional control... Well, what do you think that can handle with the bum?
Video was a preliminar KDD (ride meeting) includin many of the rocky slow tracks near Castellón (Spain).