I was wondering about water leaking through the holes! The rubber washers sounds great. I was thinking about pre-drilling as well but everything I've read about these panels say you have to be SUPER careful not to crack them while drilling. So there are suggestions to tape the area you're drilling through, special drill bits.... tons of suggestions and I'm trying to figure out how much of this I need to be worried about!
They are called roofing screws and you can get scalloped shaped pieces of wood or other material (foam ?) that fits in the groves of the panels and that really helps support the panel during drilling and also plugs up holes.