The trick to tightening a zip-tie is not to pull on the tail, but to place a long-nose pliers against the retainer and rotate the pliers to pull the tie end out. (It doesn't always work!) One alternative that has been somewhat more secure is to wrap two turns of electrical hookup wire (solid wire, insulated) around the exhaust diverter and then use the pliers to twist the ends very tightly. Not a guarantee, though.