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Another way would be to use an RFI tag (electronic chip) to open/close the locker. It could be sealed inside a little case that also doubles as your entry/exit ticket. Just swipe your tag as you enter and hold it against the door of the locker to open it.
This would have the additional advantage of enabling the locker fee to be included in the entry price. No standing there in your swimwear realising that you don't have a coin for the locker.