There's a nice stainless steel ring I have that's available in sizes up to 64 mm on Amazon. I can send you a link if you like.
Be very careful with stainless steel cock rings. Having worked in a hospital I can tell you that there's a tool that we'd use to remove them and it's not a pleasant experience. Please be extremely careful and make sure you follow all the safety instructions before use. There are numerous cases where you can get too hard and not able to get the ring off. Warning, this link has links to graphic images