I found a "backpack" combination lock at my local grocery store on their rack that had otherwise only TSA locks. I think Coleman makes it. It's shaped like a carabiner and loops through zipper holes. I think it would work well on any bag with zippers, and it feels quite sturdy (though I've never tried to jack it open). I happened to have a bag check, too, and the clerk was so perplexed by it he had me open it. Unfortunately, that didn't keep him from spreading all of my valuables out in a bin (in what couldn't possibly be proper procedure) and walking off with everything to re-xray it. I would have objected but my attention was strung thin -- my 5 year old had tripped the random alarm on the WTMD and I had my guard up about that, plus I was carrying my spaz 1.5 year old, plus my husband was off getting groped for no apparent reason. But I made a point of checking to see if he stole anything right when I got my stuff back, and everything was safe.

But I digress, it's a good lock.