Here's the back of a compact 6 mix monitor rack
It looks simple but that can be deceiving. There’s a lot of work that goes into properly wiring a custom rack. Ideally, I like to run power cables on one side, signal on the other. But there are high and low level signals. Manufacturers don’t follow a standard and position power connectors either on the left or right side of the rear chassis, which complicates things. There could also be AC and DC power runs.
I have an 8U rack, 60 feet of mic cable, and 24 connectors in the queue. A lot of soldering. Rack panels to be drilled or punched and labelled. Days will go into this tiny box.