If you use film that comes in plastic film containers, that should be good enough. Not much room for additional air in there and, if you're still worried, you could bring a roll of electrical tape and seal the cap onto the container. Luggage and clothing will get mouldy because the humid air permeates the material, condenses, then more humidity condenses on that, keeping everything moist. Film sealed in closed plastic will only have whatever humid air it carried in - probably not a significant amount.