I agree that self-tappers would be the bee's knees, but what about down the road after prolonged exposure to the elements. Are you guys putting caulk/sealant around the ends that stick through? I had thought about going the other way with mine, drilling pilot holes, then from under the bus, using neoprene washer head self-tappers and cutting off any ends that are exposed to the interior.
The reason I ask is that those who owned my bus before me went through with dry wall screws & they were rusted severely and many just broke upon removal attempts.
~(G)Q Arduously Avoiding Assimilation