I have commented about this many times here on my blog. It isn’t the right size that is the problem, it’s the ring itself! The hard edges on the rings of those knock off CB cages are what is causing your irritation.
I suggest getting a different model cage, I have been recommending the Jailhouse cage and is what I use for longer term wear. You can purchase it for under $30 with 3 different size rings here:
If you insist on continuing to use the CB cage, you can try purchasing a ring from the stainless steel model for about $10.
You can purchase one of these rings here:
I have successfully used this stainless steel ring with the plastic CB knock off cage without any issues.