Comment by chx

Colored zip ties are even better because the TSA won't have those at hand even if they bother. Or use tamper evident stickers to fasten the "tail" of the zip tie to the suitcase (search eBay for numbered, hologram tamper evident seals). This doesn't interfere with TSA but makes any intrusion painfully evident. And the trick where they use a ball pen to open the zippers, if the counterpiece is fastened to the suitcase, it's harder / impossible to close it back.