The floaty back door is kind of pain because it limits the range of motion on the mount, due to it being so thick. Plus as some have said, depending on what mounts you're using, it may not do the job.
Just ordered my second Hero3 after one became an anchor. This time I'll tether. Saw an idea for a really simple one: Use fishing leader lines with the snaps already on them. Connect around the hinge to the door and whatever anchor you need (will be a strap on my PFD for me).