And because I can't send just 1 post>
Here's a capnut in action: So you need access to the underside:
https://www.rackwarehouse.com/images...s-tracks-2.jpg
Oh, I found the rivnut alternative: It's called a plus-nut and expands much bigger but needs more space behind the sheet metal.
Also, quick bonus: Rivnuts are what is used on bicycles for accessories such as water-bottle cages.