When I'm not wearing my chastity cage, I tuck myself into a pari of tight panties. Just having a smooth front down there, encourages me to stay chaste.
Before I knew that there were chastity cages I could buy (duh) I made my own art and crafts type cock bondage. For example, I used an empty toilet paper cardboard tube as a cage. Sometimes I kept it in place with my underwear. Other times I'd rig up a complicated but primitive harness out of parachute cord.
I built my own, very similar to a Mature Metal Jail Bird, but with substantial differences. However, it's welded 316 stainless steel, and not everybody has access to a machine that can do the work, or has the skills to do the welding.
There are quite a few members on here that build there own devices, and there is a section on 3D printing in the devices section.
My wife's friend had someone who made various types of bondage furniture, cuffs, and spreaders as a side business. He was very good and even had his wife doing all the stitching on anything leather. They had a great reputation and stood by anything they made. He was a welder by trade so one would think making chastity cages would be a no brainer. The first few he made were disasters and to invest in the bending machine for metal was not an investment he wished to make. Even people who had money to burn and would buy them outfight were turned down. His position was even the knockoffs were better quality, never mind the professional and CNC made devices. He decided to stick with what he knew and now that he is retired, makes BDSM furniture full time. Like anything else, you stumble on good cages and have favorites. I have two that I wear the majority of the time, one is a knockoff and the other is more well known. Both had to be modified to get sharp edges off, but after that never had a problem with the devices. The others I have are just off with little things, a little too large, a little too small, pinch at times, or feel odd for some reason. My two cents, buy something that appeals to you and your keyholder and agree on its use.
There's no way to compete against the prices of the knock-off devices. Another huge issue is that almost no two guys have the same measurements. Almost no matter what you do, high prices for custom devices are necessary. If you could mass produce them in more common sizes, they can be cheaper, but most of them won't fit like they should. It's not like making parts for a Chevrolet engine or other high production manufactured items.