I don't know the rivet length off hand, you are just drilling through the fiberglass skin and into the square tubing so it's not too deep.
The awning should sit lower than the 3/4" down from the seal that I mentioned. This is the location to install the extrusion that the awning slides onto. The engagement point is toward the top of the awning case with the awning hanging below that point.
1/4" hardware will be used to attach the awning to the shell. Drill two holes through the back of the casing. Hang the awning off the extrusion. Mark the position of the holes on the shell. Remove the awning, drill through the shell. Put a small amount of silicon around the holes on the exterior. Slide the awning onto the extrusion, and bolt through using fender washers on both sides.