Normal pop rivet gun?
I used normal pop rivets to re-fasten an inner guard in once before.
I thought about using scrivets like you find on many local Ford and GM products. Easy to fit and remove.
I havent tried to compare sizes though.
I found some scrivets at my local fastner shop that fitted snug. I think I've got one or two left over at home. I'll get some measurements tonight (I didn't want to pop rivet everything back on since I am taking the bumper off in a month or two to repair and respray).