My boxes are plastic. They are an off the shelf adaptable box. I have seen larger metal boxes which hold multiple controllers and PSUs and are usually located on a wall or in a gaage. All depends on where you want to put your controllers and how you want to set them up.
I prefer to have mine in the yard close to first pixel, but for this they need to be waterproof. They also don't have to be large as they only hold 1 controller and max 2 PSUs as well as some fuses and cooling fans. So plastic for me works great.
I am all for the cheap $3 boxes that I have been using since my 2011 setup . The boxes don't provide waterproofing but they are cheap, can be knocked together in no time at all and with the aid of my special waterproofing agent have stopped me from getting a single drop of water onto any of the 20ish controllers and power supplies I have had running.