You could build one out of wood for that matter. It doesn't have to be of any specific rating unless you're trying to employ it in a commercial setting and have to get inspections.
Correct.
Ratings are only required for commercial settings that have to undergo regular inspections as they are required to have safe workplaces for their employees like you mentioned.
In most locations you can build and use anything you like as long as it is not permanently connected to the rest of your house. That does not mean however that it will be safe for you to use (only you can be the judge of that).
If you're the only person using a control panel that you built and designed yourself, feel free to do whatever you want.
But no matter how well you design and build something, accidents do happen so I chose to build my control panel to try and be as safe as possible. That means following industry protection standards as much as possible.
People can do what they want. They need to decide for themselves.
I doubt you pulled a residential permit to install your brew house.
I did, by the ESA (electrical safety authority) which does inspections in Ontario Canada. This was for the electrical outlets and lights that are permanently attached to the house in my brewery. There wasn't much: The 30A dryer outlet, a couple of regular 15A sockets, and some overhead lights. All installed in conduit since the brewing room was already tiled. They don't (and didn't) care about the control panel since it's not part of the house structure.
I basically paid the $80 to get an "all clear" sticker stuck to my breaker panel which in the end is really only good for insurance purposes (if something that was to happen and the house burnt down due to faulty wiring I did on this circuit, the insurance company could not say I was negligent since I had it inspected).
It was the biggest waste of $80 I've ever done ... the inspector basically said "You seem to know what you're doing... here's your sticker" without actually looking at anything. He then proceeded to ask me a ton of questions on my control panel and the brewing process.
Kal