The bolt is actually into the metal behind the plastic, I found a hole there where a bolt the same size as the one that holds the stock loop in threads in perfectly.
As you mentioned, The shelf is somewhat L shaped, so the lip is at the back, nothing ever falls forward into the passenger area... things sometimes fall backwards when I am wheeling, I have a couple bungee cords across the back now, but something usually falls anyway.
Most of the stuff up there is pretty light, spare coat, rain jacket, bug jackets, tarp, blanket... I have a couple heavier things like the hand held CB, First Aid Kit, and another set of jumpercables... but like I said it always falls out the back, into the cargo area.