The idea for trays was fairly good but proved slightly frustrating because acrylic doesn't slide so well against the PLA printed slots. I tried sanding out the slots to make them larger which helped a lot but was incredibly time consuming. In the end I just sanded down a 10mm wide strip on each of the acrylic trays. This worked great and also makes it a lot easier to see what you're doing when you fit them in.

With some holes drilled in the trays the boards were mounted on nylon spacers and fitted with some 200W knock-off PicoPSU modules.

Let the wiring begin... I needed a simple way to connect power to the tray a) without being ugly and b) able to handle the current draw from the boards. Fortunately these are very low power boards, a test run with the PicoPSU on my lab power supply showed it drawing about 1.5Amps at load from 12V, which is easily handled by a single ATX power supply with a decent rating.

How to actually manage that power for the nodes is a story for a later time...