The chassis I am working on is the 3U 400mm deep chassis. You can probably use the 300mm chassis to cut the heatsink tapping cost (because the 300mm chassis have the sink in one piece) and place the transformer and switches in a separate box for better noise rejection.
Those three boards are the power supply stack for the arduino and 12V relays, arduino board for controlling the attenuator boards, and a solid state relay for power switching. I'm planning on using the KG attenuator in this build, and I'd like to control the power switching with the arduino that controls the attenuator boards as well.