Quote Originally Posted by Madcap View Post
the correct mandrel at this point
I would think a fat hardwood dowel with a piece of sheet steel as a shim would do the trick. Could use one of the tabs from an electrical box as the shim if you need something harder.
While you are at the big box store, get a flat piece of steel, cut some squares off, and practice your technique with flat pieces before you try to do the deed on your parts. lots of light, squarely placed, taps. Your rivets will be softer than you think. Take your time setting them.