My wife's droid had a loose battery cover, mine doesn't. I compared them VERY carefully and her battery is SLIGHTLY thinner than mine. This is what made the battery cover loose.
I put two pieces of electrical tape over the bit of material in the middle of her battery cover. This seemed to cover for the battery thickness, and the door now stays on just fine. And since this is inside the battery compartment, once the door was back on you can't tell I did anything.
I tried three, it did fit, but the battery door stood out a bit. With two pieces of tape, the door cover on hers looked just like the door cover on mine. With three, I was afraid we'd leave an edge out that could catch on things, like the fabric in her pants pocket.