You could try this, cover the top of the ring in masking tape and the top of a similar sized piece of scrap (Something flat, that you can easily grip). pop a few spots of super glue on the masking tape of both sides and then bring them together. This will both support the ring and give you a better hand hold for sanding. If you use the low tack masking tape on the ring side it should make it a bit easier to get off the tape. If it looks like its gotten super sticky then you can always leave an overhang of the tape and then slip a thin blade between the tape and the ring to ease it off.

Just don't press too hard on the block as you sand, as you could crack the ring with too much pressure.