Zip ties are a good idea but the odd person that willing to do a crime will do it anyways...
One of my first trips with my friends was in the Dominican Republic and we bought locks that go on the zippers to hold them together... My friend forgot his keys at home after locking his luggage, while exiting the airport we ask employees ( specifically the luggage porters if they have bolt cutters or anything, well within 10 seconds he was already in his pocket, with pliers and had opened up my friends suitcase for us. Literally no longer than this time frame.

We were happy that he was able to help us because the start of the vacation without access to luggage can be pretty crappy, but we also realized locking the zippers together don`t do much to those that know the trick, if you lose at most zippers it is in 2 pieces, the holder that goes together with the part that `rides the tracks of a zipper`, the part that rides the tracks is normally not welded shut it just clamps down, so pliers can easily open this to unhook it from the track system, open up the suitcase, and use the pliers to close it back down. I know i suck at explaining so i will post a picture as a visual
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