it could either be just the mb on a table (thats what i got) or id put a case sideways and leave the side door off, maybe leave some mesh in place  if you want some more protection and add some fans. The deal is that these cards exit their heat from their pci rear grill, but also through some side slots, so if they are constricted without enough case fans the intake fan will be getting warm air, and the cycle would repeat itself getting hotter. On the other hand if you use too many case fans in a sealed case you could cause airflow that will difficult each video cards personal fan  (vaccuum (too many outtakes) ,jet current (a bunch of fans 50%-50% intake/outtake), or turbulent flow) from taking in air efficiently, which stresses the fans and subsequently lowers your cooling abilities.

Basically you want to renew the air around the cards as slow as you can - but renewing the air is crucial! . For instance, in my open table setup (1 6870 1 5830) i dont need any fans (although my cards are more separated than what you would get putting them four video cards on the same board), because the video card heat outputs push the air away from the intake, and the air around the intake is fresh and still, allowing the fan to work sans resistance. I also leave the door open, because, believe it or not theses things running 24-7 eventually will leave a room pretty darn hot after a few days.

good luck!