Have a look here:
viewtopic.php?f=14&t=16721462
The airflow is downwards. There's a box (exhaust plenum) underneath the unit. The metal front face plate is the height of the Origin/Onyx2 + the exhaust plenum so it covers both. There are two Onyx2's in that rack, I mounted a second exhaust plenum upside down on the bottom Onyx2 to act an an air intake. I lacked the slide rails so I mounted two shelves in the rack to carry the weight. of the systems.
Pictures of tall rack Origins and/or the handbook may have some information as well.
NB: I'm decommissioning these systems. 4xR12K @ 300MHz, 4GB RAM, IR2E graphics and a PCI shoehorn. Two units available, pickup only, 250EUR per system. If I end up parting them out I'd be happy to ship the exhaust plenums and other bits, but transatlantic shipping is going to be crazy expensive.