For a leak proof assembly there needs to be a vent that allows gas from the battery to escape (avoid building up pressure in a sealed container), there needs to be some space or a getter inside the container to collect any liquid that may come out of the battery (probably a piece of toilet paper), and also it must be approved that the container is resistant to the ugly battery goo stuff, eating it's way towards precious electronics. Some polypropylene canister or bottle combined with the help of gravity could do, as long as the mounting position is chosen appropriately and the machine is stored in according position.