Do you all lock your bags? (I would have thought most people do this) If not...
Try locking your zipper pulls together with a TSA approved combo lock. You can lock any two zippers together with a combo lock but it's probably better to use zippers made for this purpose. Many companies make luggage where you can lock the zippers (like Eagle Creek duffels for example).
Locking a zipper is not super secure but I can't think of a reason NOT to do it. You can also lock your baggage while out of your hotel room or when the bags are with the reception.
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