I had the same problem. It was a little tricky, but I got it with some patience and trial and error.

As I recall, the problem is the inner ring is threaded and then you have to deal with the nut also that holds on the closure piece. Drill out the threads of the ring with a bit just larger than the bolt. Then you have a clear shot all the way thru to the nut, which will hold everything tight. The threads are not needed in just that one spot 'cuz the nut is all that's needed.

Mark