slush cast could work. If you do, do a mold, and cast of the bottle before you mod it. then wall you do the mods to that costing, it would be easy to grind out a section of the insides for electronics (I would probably either do it at the flat end were the LED goes, or on one of the flat parts of the side, where the magnet goes, or both) and then cast it with that whole(s) in it. Then I would just either make covers for the hole(s) out of sheet styrene, either for each copy, or once, and cast them for multiples. It would be more work then a slush cast, but it could be sturdier, and it could make it easier to change the batters.
That is probably how I would do it, but then everyone says I do everything the hard way. :lol