If you wanted a perfectly round ring you could go to the crafts store and get a styrofoam ring the size you want then use glass and epoxy (Not Polyester) resin. with glass tape (4" should do) then wrap the styro with 2 layers of 6 oz. Then you could use just a bit of the same tape to make a ring to attach it to the pole where you want it. To make the ring wrap wax paper around the pole and tape it then wrap the epoxied tape around that. After it is dry you can slide it off the pole and attach the small ring to the large hoop with the same taping method. May be a bit of work but you will get what you want for about $40 of Material.

Later,
Dan