I took some copper rod - 1/8 inch or so - filed it to different diameters then did a very shallow drill on the end that created a shallow cup in the end. Mark out where you want the rivets to go, put a VERY small amount of putty there and wait about 5 minutes to let it attach to the base material well. Brush on some baby powder and dip the tip of the tool in the baby powder, blow off the excess and simply stamp out the rivets. Fast, consistent and easy. That\'s what I used on the two steam mushrooms and the Buddha.
I also did the same trick to make different sized bolts............works like a charm. Instead of using copper I made the stamp out of putty.