One way to help drainage is to watch your watering closely, giving enough for plant needs but not too much that soil is saturated and roots drown.
Without punching holes into bottom of bottle, I can only think of helping drainage by placing a layer of rocks on the bottom of the bottles before putting in soil so that excess water can drain away from soil and roots. Of course roots may want to travel down into rocks if in need of water.
Nice project for kids and challenge to come up with best methods for plant survival and growth success!
