I’m building a VIM3 cluster as well, and I came to the final call to assemble them with just m2 spacers, there was an example of this on the khadas website which seemed like a robust and compact system,
as for cooling, we might as well just add one large cooling fan, and small heatsinks to each device, to keep them thermally stable
the whole setup might be doable in under the same price range as the RPI package you prescribed, or perhaps we might need to try out a custom 3D printed solution