The beast is finished; it’s already installed in the under stairs cupboard and playing away.
Here are the finished pictures just before the cover was fitted. The 4 switches on the front are individual power controls to the network switch and Pi’s; this allows for just a single Pi to be switched off for a reboot or to change the speaker cables etc.
Very pleased with the results, and it sounds better than the Sonos ZP100/ZP120 it replaced for a fraction of the price.
I’ll will work out the final build cost for future reference.