Don't put any electronics on anti-static bags.  They're designed to protect their contents by pushing the static electricity to the outside of the bag.  If you don't feel like drilling a few holes into a sheet of metal or wood and screwing in some standoffs to make a ghettofied motherboard tray, just set it on something like a piece of cardboard or even the box that the motherboard came in.



That's actually what I'm using for my rig.  I was going to build another full computer anyway so I figured I would get an actual case for everything and just sell off the extra cards when I'm done mining.  That case has a fairly sturdy plexiglass-like covering on it so I have two cards in the main motherboard x16 slots and the other three sitting on the top of the glass towards one edge sitting in the removable gpu bracket that came with the case, and a second PSU powering them sitting on the opposite edge.  I'll probably have to come up with a better long term solution though since there's a bit of sagging going on, but for now, it's working out pretty well.