I just posted the photo to the other thread about jingling zippers, but I'll post it here, too.
It's a bit of a tight operation but this works real well:
PS: Added bonus. At least on things like the Packing Cube Shoulder Bag (which the photo is of), the loop you get from the zipper pull forms enough of a hole that you can quickly clip it to the snaphook of your shoulder strap. Sure, it's not the same as locking it down, but if you're on a quick bus trip in Rome and you just want to prevent pickpockets from getting into your bag while you're carrying it, this will work really easily. Plus, you aren't removing the original zipper tab, so the process is reversible.