I saw that thread where the guy noticed his battery was loose in the tray. I went and looked at mine and, sure enough, it was loose too.
The clamp is plastic with some ridges on the front to grip the lip of the battery. With the screw completely tight, the battery still slides back and forth freely. So here's what I did for a permanent fix.
Piece of vacuum hose (about 1/8" id??) cut the same length as the clamp. You can kind of see the ridges there, they were worn from the movement.
Put the clamp in place with the hose under the front lip and tighten.