GoPro camera (insulated in a box)
From some recent gliding experience, where we did not go that high to begin with (just to the bases of the clouds, to be precise): Use the open back door, or use some anti-fog inserts. With the closed back door the camera will fog up.
We had three GoPros attached to a glider just two weeks ago. All outside the fuselage. Two with open back doors had no problems at all, but the GoPro with the closed back door fogged up rather rapidly.