Right, you should definitely prepay. But if you don't use the locket system then you have to remember to go and get the box. The nice thing about the locket is that it's always there around your neck, so you don't have to remember that the box exists or the date (which is engraved).
// Prepay for 40 years, leave the message in the box, forget the lockets. The bank will chase your for an extension for the rental of the box.
Far from foolproof but if you're already relying on a deposit box ...