Is it necessary to screw the lid down of the battery storage container to completely achieve a waterproof seal?
To achieve the waterproofing, you will need to tighten the lid down as far as you can. It is hard to say for sure if you can have it just less than fully tighten but you can actually test this if you wish. You can simply test it by making it fully dry and then tighten it down and submerge it under water to see if water seeped in or not. If it did, dry it off completely and tighten it a bit further than you did previously.