I got 2 suggestions for you, and both I still use.
first is find a hardware store that sells rubber surgeons tubing like the stuff on a sling shot. its kinda yellowish, maybe 1/4 inch diameter, hollow tubing really stretchy. buy a couple of feet of that, its pretty cheap and that should be good for several. at the same rack that you got the surgeons tubing buy a short piece (12" or so) of that clear vinyl tubing who's outside diameter is a bit larger than the inside diameter of the surgeons tubing, test different sizes till you find a good match. also get some super glue. now take a length of surgeons tubing that makes a loop about 1-1/2", then cut a short piece of vinyl tubing maybe 1/2". put a few drops of superglue in the inside of the surgeons tubing at one end and quickly insert the short piece into the tube with the glue about half way. do it quickly cuz that glue sticks quick. now do the same on the other end so you form a loop. once you do a few you'll get better at it. make a few of different sizes and you'll figure out which is best for you. best thing is, they are cheap, easy to get on and off cuz it stretches and you can make any size you want.
second is my current favorite. buy online or at a crafting store some stuff called coolmorph plastic or polly plastics or polymorph there are several brand names. It is Hand moldable thermoplastic that has a low melting point of like 110F. you can buy it in strips or small beads, and you heat up some water to almost boiling and pour it over this plastic and it turns clear and is moldable like clay until it cools down and hardens. it is re-usable as many times as you want and can be formed into just about any shape you can make. its extremely durable and strong when set. its awesome stuff, I have found all sorts of random uses for the stuff. anyway, melt up a batch of the stuff and roll it out into a thin long piece of whatever diameter you want you ring to be. A good trick for making a consistent diameter is find two magazines the same thickness place them on a table about a foot apart, roll some material into a tube shape with your hands to start it and get it round, then place it on the table between the magazines and with a large flat object, like a book or chopping board, start rolling out the shape. roll it out until the board or book is down to the top of the magazines and you get a nice even snake of material all the same diameter. you will need to re-heat the plastic a few times by placing it in a bowl of hot water to soften it back up, form it into a ring and cut it to whatever length you want, heat up each end and mold it together to orm a closed ring and and there you have a custom cock ring.
best thing about both these is that you can make many, many rings, custom fit for you for way less than the cost of a single cock ring from the store. here are just a few of ones I have made to give you an idea. also a few links to products. good luck and happy cock ringing.
https://www.amazon.com/Spearit-LATE...keywords=medical+tubing&qid=1599925132&sr=8-2
https://www.amazon.com/Polly-Plastics-Cosplayers-Hobbyists-Accessories/dp/B00WRWKM6G/ref=sr_1_3?crid=2YF0DX7VEIA6S&dchild=1&keywords=polly+plastics+moldable+plastic+pellets&qid=1599925020&sprefix=polly+plastic,aps,183&sr=8-3