IMRbatteries.com sends battery cases for free along with the batteries. If you are in a pinch for time, you can use an old pill bottle that is plastic, but only one battery per bottle. You need to keep them isolated from each other. I also wouldn't want them to be rattling around in there, so stuff the case with foam or paper towel to make sure everything stays snug. If you have a vendor (brick and mortar) near you, it might be worth calling to see if they sell battery cases, as some of them do. Same precautions needed, anytime, so a battery case is useful if you are ever transporting any extra backup batteries while out and about. I do carry spare batteries in case I need them in a pinch, for the most part they remain in my purse but safely enclosed in a battery case. I remove and replace from time to time, to make sure they don't use charge as it's rare for me to actually need them.
Have a great flight!
Anna