carg Yeah, they are designed to accept standard ME/2-style locks, like the HolyTrainer's magic locker. Alternatively, it is possible to design padlock attachments for it, but generally, locks are always an ingredient you buy, as it is pretty close to impossible to design something to be reliable to use, yet difficult to destroy with a printer. Plus if the key designs were public, it would be easy to copy the key. Another component you need to supply is some food-safe (or at least body-safe) surface sealing material like clear coat or epoxy, if you are interested in long-term wear.
As for comfort/security:
Printing a published design makes it easy to experiment with things like cage and ring sizes, though the number of free sizes has dropped significantly since I downloaded mine. A larger selection of designs is available on the designer's patreon profile. Took me a bit of experimentation to find the right ring and cage size for me. The designer has guides for surface finishing (3D prints worn on the body for prolonged periods absolutely need that), and recommends sandpaper and a sanding attachment for a rotary tool. I have found that disk sanding also helps a lot for the outside. I have no disk sander, but a small sanding attachment for an electric drill worked wonders. I definitely recommend doing at least a coarse sanding before wearing it for a few hours to check the fit.
After I found a suitable size, I have to say it is a lot more comfortable than other printable chastity cages I have found, and, I feel, somewhat sturdier as well. The EasyCB is only available in a limited range of sizes, the parametric cage no longer has the base files available, and the New Fun, while being a direct predecessor of the Cherry Keeper, really suffers from the limited resolution of filament printing and quickly broke for me. The KB3D cage has held me under lock and key for weekends with only minor discomfort. Not really something to write home about considering some people wear their cages for a month with no break, but I am taking things slowly.
As for security, I have been able to pull out, and I have been able to shake the cage and orgasm that way, but both have also worked in the other printable cages I have tried. My penis is able to turtle up completely, which makes pullout almost impossible to defend against without a piercing lock, urethral insert or cage pushing into the body, and I consider none of those options, so I will take what I can get.
I think by now I have spent more time and money on those things than I would have spent on a HolyTrainer knockoff, but honestly, the fact that I got a craft project out of it and not merely what ultimately boils down to a sex toy (even though it is more of a not-sex toy) is something I see as a plus.