How about a cable lock? Normally I've used them for securing ski gear but they might work in this scenario.
http://www.ebay.com.au/itm/Retractable-Code-Combination-Cable-Lock-Bike-Luggage-suitcase-Security-padlock-/252935724716?var=&hash=item3ae424deac:m:mz0n1Mwx6NG0H9SQslYBu0A