I know you probably don't want to, but what about removing the covering from the first lightening hole (on the bottom) behind the wing? Strap the battery to a length of wood that runs from behind the the first former aft of the wing to the second one aft of the wing and glue in place between the two formers... you'll wind up with a patch in your covering, but at least it's on the bottom where it won't be seen.