The design of the tray is completely wide open at this point. It could be as simple as a wall of boards, all cooled by a simple box fan:
Or something more modular. This design is expandable in every dimension:
You could mount boards on both sides of that center "blade."
The use of t-slot extrusions removes all possible issues with board size, separation, connectors, etc. because the entire case/tray is almost completely adjustable.
The "modular blade" idea could even be used inside an enclosed case, where when blades are added, the spacing between blades could be easily adjusted for optimal cooling.
Once again, this modular design is
very flexible: