How difficult to file fit rings ?

They sell a tool for filing rings. If you don't want to spend the money for the tool and work carefully nail files from the pharmacy work well. Just support the ring on a flat surface with the gap just hanging over the edge. Place the nail file between the ends and let the ring pressure against the file do the work as you pull the file up and down. Make sure to clean up the edges when you are down and check the gap often.