First design the size, and then take a single panel from Taobao and let the seller cut the size for you. Buy a set of tools, such as punching and milling. First, punch holes in the back plate. Basically, the hole position of the motherboard is standard. Find several motherboards with different specifications and try to fix them with long screws. Then you need to consider the fixing of the power supply and the opening of the socket.
Everything needs to be put together except the side panel. I use angle code to fix it, and the chassis factory usually uses rivets to fix it.