Here can be seen my effort to temporary stop water leaking into the battery. This temporary "Fix" is now almost permanent. Any water leaking down is drained into the tray which the battery stands on. This tray has a drain pipe to the outside world.
The temporary "Fix" is a plastic 5 litre bottle cut in half, and secured with an elastic Bungy. There is enough space around this cover to allow the battery to exude it's fumes.