If you are going to secure things in place with only a few rivets I would use washers on the rivet to increase the surface area supporting the cap. This will aid in preventing the cap from cracking at the rivet.
The number and placement of the rivets is a hard one. I would aim for those rivets that will be in shear around the bottom edge, to take the weight and some at the top, to stop possible rotation. a dozen should be enough for that distance.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles