I made one this week. I laid down and put my toes against the wall and put a pencil at the top of my head to determine length. Then I designed it on paper to be 36 at the feet and 48" ay the top.
I went with two layers of Primaloft One...
1.1 oz coyote brown for the inside, very soft, and 1.9 downproof black for the top.
Cut it, set it brown, black, insulation. Pinned it and sewed 3 sides. Turned it inside out by pulling the brown around. Then sewed the foot with a channel for shock cord. All that should be left is to put snaps on the foot box.
I say should, because I'm going to reopen it and put in another layer of insulation. Cost so gar about 66$ and 4~5 hours.
I think 48" at the top is to wide.