Last night I tried out a pair of Alpha Direct booties that I made. The low temp was 32F, 0C, and I was under an apex quilt rated for 30F. The booties were very warm and comfortable. The booties replace a thick pair of wool socks which I would usually use to keep my feet warm.
The design was a simple rectangle, folded over and sewn to make a tube, with the end sewn shut. I did not try to shape the booties into a foot shape, they are just a straight cylinder. The length is from the outstretched foot (toes) to the bottom of the knee. The width is the circumference of the calf plus an ample amount (50%) to create a loose fitting sock-like bag that fits over the lower leg. I inserted a thin elastic cord tied with a light tension to hold the top of the bootie above the calf.
The booties were very warm and comfortable, far better than a thick wool sock.