I have no clue if it's still available, but a while back you could get up to 3 backs for free, and I ordered mine up right away. The new one held on a lot better and I only replaced one because I got some super glue prints on it. So like I said, I'm not sure if it's still around, but maybe search to see if you can maybe snag up a new one.
Also, you could try to bend the metal gap (with a flathead screwdriver or something similar) where the battery cover indentation sits. I did this before I got my replacement backs and it seemed to work for a while, but needed to be re-bent every so often.