I purchased a generic metal cage online after measuring numerous times. I measured for my ring multiple times throughout the day and averaged out to be 47mm. My thoughts were to go a touch on the small side to insure a good fit so I ordered the ring in a 45mm. My cage was measured the same way and I settled on a 45mm also. The ring was tight getting on but not unpleasant and fit snug at the base allowing a pinky finger under up the first knuckle with slight persuasion, the cage fit great. The ring did cause some irriation on the first couple of days but nothing major or unbearable. Now on day 3 of use the ring is slipping down away from the base and the cage is getting 'loose' allowing for some slippage and uncomfortable pressure on my testicles. I initially thought the ring was to tight but now I'm not sure could it be too big? What do I do?
Get yourself one of these, works great to keep it all nicely to your body: https://www.etsy.com/listing/790431887/custom-made-leather-cock-ring-and?
The cage may be too heavy for you. I bought a number of cheap metal devices some years ago and although they were similar ring sizes , some had much thicker metal and consequently they weighed a lot more and would slide down. Perhaps try a plastic or silicon device and see if that's better. I am a huge fan of the Male Chastity Now Contender which is metal but almost as light as plastic. It's custom made so costs more but if you get into chastity for the long term, I'd recommend checking MCN's website. Other CM members I'm sure can suggest their favourite low weight devices too.
It could be the combination of both, the cage is to heavy and your ring is to big, first try downsizing your ring and see if that helps, if not maybe lol for a new cage? Good luck.
I had a similar problem with a cage i loved but the main issue turned out to be the fit between the connection posts of the cock tube and the ball ring. It is too loosey goosey, it has too much play between the large ball ring and the small cock ring, and didnt constrict the cage against my testicles enough. Those are the keys to keeping these things in place for the most part. Too wide an aperture and the whole apparatus starts sinking towards your knees. And it lets you balls squeeze thru and hide in your body cavity. The strap shown above may be the best way to salvage this device, and believe me we all go thru devices before we find the right one. The one i have on now fits together with machined perfection. No wiggle room or rattling. Plus its not a circle its a bent oval sort of and that helps keep my delinquents incarcerated and tight against my body.