Back ring help

I've recently bought a bondage masters chastity cage from Amazon but it only has one size ring. Am I able to get a smaller ring that will fit the lock properly. I need the next size down so maybe a 40mm without having to buy a whole new cage

https://www.amazon.co.uk/Bondage-Ma...ge+Masters&rnid=1632651031&s=drugstore&sr=1-7