This is a simple cinch bag made out of polyester fleece (banana for scale) . Its purpose is to wear on your feet while in your sleeping bag while camping.
Do you get cold at night while camping? If so, what part of your body is coldest? Your feet of course.
As odd as it sounds, your feet will actually be warmer in your sleeping bag if you don’t wear socks, as your feet are then able to keep each other warm. Just like wearing mittens instead of gloves.
And this little bag helps keep them even warmer.
Last year, I tried this bag out when camping with my scouts. I had my 40 degree summer sleeping bag, a stocking cap, and this foot bag (monosock?), and I was OK. Not toasty by any means, but I was fine.
I am especially pleased with how small it rolls up to. It has become a permanent part of my camping gear.
While I made the original one last year, I’ve been fine-tuning it because my Boy Scouts want to make some themselves. We’ll all be making them together this week.