Putting them in a cooler is a good idea. Frozen bottle of water is a very bad idea -- lots of condensation which will raise the RH.
What do you expect your ambient temperatures to be during the heat wave (both highs and lows). It may be that your cigars will be fine for the duration.
I'll just keep them in the humi and put that into the cooler with a frozen water bottle, it's small enough to fit.
__________________ "Man's mind is his basic tool of survival. Life is given to him, survival is not." -John Galt