I live in the middle of the Canadian Prairies, and fly several times a week. The average temperatures on my flying days are around -20-22C, with the coldest day of lying I've done at -33C. I have a small, one man hunting blind with a built-in folding lawn chair...I guess in actuality it's a lawn chair with a hunting blind mounted on the back of it. Regardless, I set it up, and put my little Buddy propane heater in it to keep me warm and out of the wind. I open up the main front shooting port for landing & taking off, so I can visually monitor it while I put on my Fatsharks ( this time of year I fly mostly FPV).
You can avoid fogging by letting your camera acclimate to the outside temperature before you turn it on to go flying. It's very easy to take out the battery on the go-pro, so I do that to keep it warm, then set things outdoors for 10 minutes or so first to "cool down" Once it's cooled off a bit, it won't fog up nearly as easy. If you're still getting fogging problems, take a dryer sheet (Downy, etc) and wipe the inside and outside of the lens port on the GoPro with the dryer sheet. It will keep things from fogging up pretty well. You can also use a small bit of shaving cream; rub it on, then buff thoroughly, it will keep fogging under control very well.