A set of luggage strap (better if 2) will take care of your problem. Since I like to over-stuff my bags, I do this as a habit. The cost is $1 (dollar store or ebay) to $5 (Walmart). It also make your luggage a little easy to identify coming off the belt, and slightly less desirable to casual thieves (luggage straps = overstuff = difficult to stick hand in to rummage)