You are probably going to use 1/4" pop rivets so I would tape the ring in the location that you want and then drill one, or all four holes, in both the ring and the sidewall. After you have drilled the holes, you can insert a straightened out paper clip, or other stiff wire, with a little bend on the end. Insert the wire and hook it on the inside lip and use your fingers to hold it at the edge of the ring. Pull it out and you have your wall thickness. Then head to Home Depot, Lowes, etc and get the right aluminum pop rivets.
Be sure to check your location so that you are not drilling in to any frame members.
