You've given no idea on the actual size or weight of the battery pack these tend to be heavy, & down tubes, or set tubes where the water bottle holes are, the tubing is normally round, so no mechanical support other than the screw holes, & we all know the weight of a water bottle with liquid, so going over bumps, vibration could be stress here its not designed for ? Rather than making holes in any tubing, try initially to use strong cable ties instead making extra holes, see then what you think.