Have a few of those k edge replica style mounts from Amazon. I've used a bit of inner tube to ensure the bracket stays put, but if I go over rough ground the camera itself ends up moving. Any way of increasing the friction in the camera / mount interface? Tightening the bolt gets to the point it feels like it is going to strip and still slips.
Filling the alloy and carbon paste an option?
It's an obvious point, but try a k-edge mount. They don't slip on the bars and they clam the gopro harder. I suspect the slot tolerances are to great on your mount. Mine tend to be pretty tight to slide before tightening.
Try carbon fibre grip if you've got it, might increase the friction when it's clamped up?
I looked at k edge but the prices are a lot for a mount. I'll give the paste a great before I pay out £30+
Toothpaste?