I have small wood rails (made from 1/4 round) with thumbscrews that can be fastened to the end of the cards but I don't always attach it because I'm constantly tinkering and improving the layout, swapping power supplies, etc.  I also have 2 ducts (not pictured) that I keep over the cards that channels the air from the fans in the back.   I don't have any problem with them sitting they way they look in the pic though, it's flat and the cards don't move.