Fogging is a real problem with GoPro cameras, particularly in diving where humidity is so often an issue. Closing the camera in a damp tropical environment, then submerging the camera in cooler water to dive a tropical reef is a sure recipe to fog the inside of the optical window.
There is plenty of room in my other camera cases for desiccant packs. I keep a good pile of desiccant on hand in nice little packets just perfect for loading into the camera cases. I put the used packs in my toaster oven for an hour at 150°F to bake the moisture out and reuse the packs over and over again. Simply store in a tightly sealed, airtight container for use.