I wanted to take some rear facing videos and pics from my Harley but the GoPro handlebar mount that works well on my passing lamps bar up front won't work in the rear. The rear luggage rack was the obvious place for a mount but I didn't want to stickie a flat GoPro mount on the rack because I often carry things back there. I tried two versions of mounts on the rear rack and both worked well.
The first mount is simply using a GoPro flat mount and two cable ties. I just cross tied (real tight), using the cable ties, the mount onto the rear rack. It was very stable even though I left the stickie pad (with cover) on the mount. A mount without the stickie pad would be even more stable.
For peace of mind, I used a safety lanyard (see my post on Safety Lanyard). The red arrows in the pic below help you see the black lanyard against the background.
The second mount (I like it the best) is using the GoPro camera shipping mount (the bottom of the plastic housing the camera came in). I drilled four 1/4" holes in the shipping mount (placed appropriately to allow cable ties to go around a cross bar of the rear rack). Then, using two cable ties, tied it down. Because of the large area of the shipping mount, this method is very stable. The red arrows in the pic below show where the four holes are (the wire ties are already in place). The yellow arrow shows you the round cross bar of the rear rack that the cable ties go under.
Again, for peace of mind, I used a safety lanyard. The red arrows help you see the black lanyard against the black shipping mount.