Over the years my Wife / KH / Goddess and I have experimented all sorts of cages: metal, plastic silicone, cheap and expensive; and in various shapes and sizes and even adjustable like the early CB-2000 with spacers and rings. My problem is not especially with being to pull-out-the-back but rather one of testicles slipping back through the back ring. Once this has happened firstly I find it is painful (not a good look) and it is effectively possible to just pull the entire cage off. The first time it happens to me it sort of ruins that particular cage - since I know it is not secure. It has actually happened a number of time during the night when I guess I am relaxed, warm and asleep and I don't even know about it at the time. I have experimented with different size back rings, from 45mm - 50mm. 45mm seems too small to be comfortable for long-term use and 50mm a bit too loose: although the ergonomically shaped back-rings too make a difference. Although it's an issue with all shapes and sizes. I used to think that the closeness of the back ring to the cage itself was the key factor: the amount of room to squeeze back through but now I'm not so sure. Is this a problem for other people and has anyone found a way or a cage to solve the problem ?
Have a look here https://www.customchastity.com It to a couple of prototypes but I now have the absolute perfect fit
I had the same issues of escaping balls, and ball pain. I tried everything, including modifying my CB3000, and having 2 cages, with different rings, one bolted directly to the base ring, etc. A few months back, I bought 2 Chinese metal devices from Amazon (with built in locks), both with 50mm rings as I'm too thick when hard for 45 and under. One cage is just a vented cup essentially, the other is a short tube, with a barred end, and an oval shaped base ring. Both are comfortable, both have not let my balls escape even once, and no ball pain. I have some problems with extra skin getting pinched, as they are smaller cages, and I'm not, but I've had zero ball issues!
I don't think it is a ring size problem per se, but more of a ring shape problem. Unlike the Holy Trainer version 3, most male chastity devices cuff rings are not anatomically designed, whereas they should be more triangular.
There's two parts that work together to keep your genitals in the device. The first is base ring size. It should be snug but loose enough that you can get the tip of your finger between the ring and your body. It should never be tight enough to cause swelling, skin cooling, or skin discoloration. If 50mm is too big and 45mm too small, then you may have to look at a custom device. The other is called "gap" and it's actually a three dimensional space created by the area in the base ring less the area of the tube plus the space between the front of the base ring and the rear of the tube. With the same base ring and tube sizes, the only thing left is the angle at which the tube sits to base ring. Custom manufacturers can adjust this and Chinese devices never mention it. If you don't want to go custom, then you'll have to try different devices until you find one that's right for you. The CB-6000 series makes it pretty easy to adjust the space between the base ring and tube.
I've used the CB series for years, and always ended up with either escaped nuts, or bad chafing. For some reason, for me, with the 2 Chinese cages, when my balls try and pull through, there isn't the pain and cutting in that the CB series had. I had some luck with a zero gap design, using the 2nd largest ring that you had to special order on their site. In the end though, they always snapped where the 2 posts go through.
Plastic tends to soften when kept hot,and your crotch is generally a hot area. I had the same problem with a Holy Trainer and slipped a ball while at the gym. It hurt. Steel doesn't give. Get the right size and you won't have a slippage problem. I looked around and settled on a the Jailbird from Mature Metal. Before you buy they also sell plastic sizing rings you can try, and they help a lot in making sure you don't have to send it back and forth several times. You try a ring for a few days. Then you size down and give that a try (even if it feels tight at the start, give it a few days, you body is pliable and may adapt). Rinse and repeat until you find the size that's right for you.