[topgun]

Think about what the rivnut is doing, and how you can make your own tool.

Drill a close fit clearance hole for the size you are using in a piece of thicker steel. Put a bolt through the hole, screw the rivnut on the other side so it's tight. Place the rivnut in the mounting hole where you want it and use a powered drill to turn the bolt. Tightening the bolt will pull the rivnut bending portion tight to the mounting hole.