Just a heads up - these things are really heavy! Plus it makes people wonder what you have in your backpack that is so valuable...

If you need to lock your backpack to something (eg. bed, train rack, etc), I recommend this cable plus lock instead - very light, inconspicuous, and can double as a laundry drying line if you have hooks or little suction cups.

Cable and lock

In reply to the original question, my travel backpack has a panel that zips over the shoulder and waist straps so nothing is loose. For hiking packs, I'd use a large garbage bag or two - also good for a rain cover.