I use cable ties in an assortment of unusual colours to keep zips closed and on Pelican cases. It's not that they'll stop someone getting in, but I'll be able to tell if they have, as the perpetrator is highly unlikely to have ties in the same combination of colours. I obviously don't keep the spare coloured ties in my checked bags but my carry on.

I've also modified zipped luggage by putting eyelets in so that a cable tie secures the zips in position and the zips can't be run backwards and forwards to reclose a compromised zip.