I’m not sure this has been suggested before, but I thought about what happens when the router slides out of the z-axis arm and slams to the plywood due to the bungie tension and thought that it probably could be prevented with a slight tweak to the sled. The idea is basically put ‘wings’ on both sides of the center hole and install a couple of wooden dowels so that if the router body does pop out and heads toward plywood, it would hit these stops first before the chuck hits the plywood. Not to scale. The two small holes are where the wood dowels get installed.
I’ll probably try this out when I build my new sled.