The build of the box is just about complete, I just need to do some cable management, paint the exterior and fit some latches on the doors.
A big development in the design is the power management. I was originally trying to fit the power supplies for each devices into the bottom of the unit, but after doing some mockups I found it was taking up about half the space required just for the PSUs.
To overcome this hurdle, I have enlisted the help of a couple of adjustable output DC-DC step down converters that I got for a few dollars each from Aliexpress(pics below). The highest voltage device is the 8 port PoE switch. This uses a 48v 60w PSU. I have ordered a 48v 120w PSU to replace this. I have installed 2x step downs, one that takes the 48v to 12v. The 12v can then be used in the modem and the extractor fan. The 2nd step down is set to 5v which can be used to power the Raspberry Pi.
Having installed the 2 step down units I am happy to report it all works really well and the whole unit stays very cool thanks to the slightly over-zealous fan mounted in the back.