Youve already put a lot of work into it, I'd go with what you have I think you could find a small plastic box, lets say the kind that holds screws or something and make that work. A lot of guys in the Crawling websites are using similar plastic boxes to waterproof their electroncis and batteries. I would go as far as building one out of balsa and covering with fiberglass. Thats what I do for my airlplanes when im wanting to protect my lipos in case of a crash. You could also build a box out of sheet lexan or acrylic from Home depot. Screw it together or use that special proweld adhesive and it will be super strong and clear so you could actually see the battery, then just fit a holder over the top.