I have a ‘palm’ and also made it much as described above. On my first one, I used a stainless steel 3″ diameter ring. It worked well, but banged on the deck from time to time. On my second and all subsequent ones, I have gotten a section of 3″ diameter PVC pipe and cut a piece off about 1/2 to 3/4 inch long. I filed the edges to make them reasonably smooth and attached the shock cord with a hog ring and now have much lighter and ‘quieter’ palms.
These are easy to make and don’t cost much and do prevent the mainsheet from ‘snagging’ on the corners of the transom, especially when jibing.