This is a fragment of the article which looks promising. A comparator to monitor battery voltage and a Mosfet to switch on and off. Full details are in the magazine and you can order a Printed Circuit Board from them:
The design is for two batteries but it also shows how you can reduce the components for a single battery.
The on/off switch could also be useful for model submariners. A normally closed reed switch and a magnet will form a handy external switch.