[Funny]

I wouldn't use rivets, they are far too permanent for the application of a belly pan. What happens if you need to get to the underside of the vehicle for a repair, wasted minutes of work removing rivets and possibly damaging the belly pan? I would use self tapping hex head screws and fender washers, they are less likely to damage the belly pan, are easier to take off when repairs are needed, and with the fender washers, are more likely to hold the belly pan against the body without the pan getting pulled off (like if you bottom out).