Brad,
Find a scrap of sheet metal and drill a few holes in it and have a practice until you are confident with the way the rivets work.
There are various another products that can also solve your problem. There are nuts that can be crimped into sheet metal with a rivet gun, others that just get hammered into the sheet.
This might work for you depending upon the clearances between tube and cell. I'm not sure because although I know the type of reflector you are talking about, I am not familiar with the cell arrangement.
https://tinyurl.com/2r7crw5s
Joe