I dont use one. I dont know if they exist.
Mine is a square about 35 inches a side, so to make that I would need a piece of cloth 70in by 35? yes?
Folded over and sewn one side.
So, you have a pouch joined two sides. the apex corner of that you cut straight across big enough to go over your head.
The hood is a rectangle doubled over (measure on your head.) and sewn on (this bit is hard to describe, it makes sense to fiddle with it on the cloth)
HOWEVER
This is using woolen cloth; if you was using cloth that hangs neater it would need to be shaped more to fit on your shoulders. (Like when i made a similar wrap of silk)
Hope this helps