The whole point for a rack mountable case, is so you can mount the cases in a rack. I want to be able to mount 10 4U cases in a rack. You can't colo motherboards on a tray.
"space efficient"
Who cares about space efficiency? The only thing that really matter for mining is cooling ability (IMO).
What? Most data centres allow you to colo non rack equipment on steel trays. Considering that you're paying for space, of course space efficiency is important. I pay half the price per rig when I squeeze 2 rigs into a 4U rack. Cooling ability in a DC? Just let it rip with some 5000 RPM fans and not worry about the noise.
It sounds like you haven't thought this through carefully.