I really like the design of CBs where the cock cage extends a bit behind the ball ring. For example this one. https://www.ebay.com/itm/Electric-Male-Chastity-Cage-Electric-Plastic-Cage-Ring-Ball-Lock-Electro-Device-/125008245138 - I think the Holy Trainer does it too. Has anyone experimented with a design where a lock would be on the bottom of the ball ring instead of the top? the cock cage would need an extra piece to go under the balls and attach to the lock. I think this would reduce the appearance of the lock bump with tight pants and could be more discreet.
Steelwerx Extreme has a number of customs that I've seem that include a base ring, a ball cuff, and the cage is attached to the ball cuff. It took me a minute to figure out what was going on, but it's quite an interesting look. Many of their cages go together with security screws, so there's no lock body in those cases anyway. EDIT: noting the forum this was in: I think this sort of design would suffer a bit in how it works due to the clearances needed for an actual lock, and the flexibility of plastic, so it would be a very difficult design to get to work
That's one of the many advantages of a steel cage, because steel is much stronger than plastic, the components can be made much thinner, so the integrated lock on top on many steel cages doesn't cause a bump at all.
Here's an erotic fantasy I have is to have is to have the padlock swinging behind my balls tapping them. It would be less visible and yet be a constant reminder.
That could be easily engineered... You'd need a piece of stiff steel wire the same diameter as the padlock shackle to make a retaining pin that replaces the padlock at the top end, with the padlock now securing it at the bottom end. The top end of the pin will need to be plain to fit into the normal locking hole, and have a curve or two at the end, so that once that plain end of the pin is pushed through, rotating the pin to connect the bottom end means that it can no longer be slipped out of the hole. At the bottom end, a loop gives a place for the padlock to secure it to the bottom of the cage. Something like this (the new locking pin in blue)... Ok, this design will have the padlock tapping the front of your balls, but you'd have the same effect! To get the padlock behind the balls, the pin could instead run alongside the base ring. The bend of the hook at the top would just need to be tailored for the particular locking post so that it couldn't be slipped out of the locking hole while the padlock is in place. For this design, two pins, one each side would work well. It would look balanced and you wouldn't be able to slide the padlock around the base ring.
So here's my idea is where you have a kind of a swing latch at the top of the ring where the cage swings down. There be a what I would call a locking rod that goes from the tip of the cage and inserts through the bottom of the base ring. The padlock would be locked behind that.
The location of the lock is not important, it need to be very small and tamper proof. I like the steel Nub but for me the best cage is the little device that compresses the little clitty into the body. See photos in profile page.