I've settled on a way to add the boards for my cluster; I've added two rails for strengthening on the back of the frame, and the gap between the two is wide enough for the Pi (mounted to it's own small frame) to slot in. I've made the top bar of the Pi frame 1cm wider each side so it rests on these bars, and I'm using an angled bracket on each to allow me to screw it into place.

Nice and solid, and only takes 1 screw to remove it.

Bonza.