I saw a youtube video where a guy used 2 pieces of cardboard to snug fit the HDD... then he used a metal screw clip to hold it in place. It was good a snug. Im at work so I cant get to you tube right now.
Im thinking of doing the same thing. I have a broken ps3 with a 500g. I'm going to get a 12g model and just let the hdd float. I dont move my PS3 around.