I already had everything needed to make one except for glue and oven bags, so I gave it a shot. I tried out my slapdash version today with nothing in it. It got up near 200F in less than an hour and maintained that temperature for much of the day. I'd say it should work decently as a slow cooker. Also, it gives us a way to cook without using up fuel (propane and wood are our other non-electric options).

I should emphasize that this first attempt of mine is a very crude one. At one point, I even cut the flap on the wrong half of the panel (entirely my fault for not paying close enough attention to the video -- just follow what she does). A better made version would presumably work even better.

Also, for what it's worth she has a subsequent video where she manages to bake bread with hers, and this was on a cloudy day in (I think) Utah. Now, that bread won't win any awards, but it is edible.