Some luggage has a built-in TSA-approved lock that you put the zipper pulls in and it locks them in place. So, they'd still be able to get the zipper open, but then wouldn't be able to close it back up, as you noted with attaching the lock to the handle. Of course, you'd have to buy new luggage for this.

If you're traveling internationally, you might not have an option, depending on where you're going. Some countries participate in the TSA lock program, others don't.

If you're really worried, you can try shipping your luggage to your destination, if your hotel or other destination allows for it. I've heard of people doing that but have never tried myself. It isn't foolproof, but eliminates any airport shenanigans.