I have heard people using both rivets and screws but i found that when you do that the pannel does not slip over and down flush on the body lip because the screws or rivets will hit .
What i did was got some very heavy roofing staples and took my time clamping the fussy in place with small welding pliers and small electric drilled the holes for the staples and crimped them as tight as possible like the factory did using another set of pliers .
Worked perfectly , just like the factory i even found that if you stagger the staples the it holds them tighter . Cj