For the frame I was hoping to keep the shape similar to the Totem Mini Lab I have at home.
Now the switch has been chosen I've had to make some changes to allow it to fit; I've raised the front gap by a centimeter to allow full access to the ports on the switch, extended the back by around 8cm to incorporate the switch's hefty power supply, and will be adding more beams to brace the frame.
I'm going to cover the gaps either side of the screen with some panels, leaving a small gap to route the ethernet cables through to where the Pi4 boards will be mounted. I'll also make a cutout at the top of the frame for it to be used as a carry handle.
Regarding mounting I am going to mount them with the ethernet & USB ports facing upwards for easy access. The boards will be mounted to the added bracing area stacked left to right