I bought the rivnuts and the assembly tool from the same place. Several places had installation tools that cost $70-$90. The one I used was only about $10. It's really simplet. It's a large hex nut with a slot in the side of it to hold the rivnut in place and then a hex head bolt that pulls the rivnut tight and collapses the wings of it. See this link.
Plusnut Blind Fasteners - Find Plusnuts Fasteners at Cardinal Components
When I used the tool and compressed the nut it pulled up real nice and tight. I was able to go through the c-channel as well as three layers of aluminum, so I feel like it's structurally pretty sound.
I've been helped so many times on these forums, it's nice to be able to help somebody else back.