Finding the optimum raw material might be hard and expensive, yes, particular if you have to ship internationally.

Can you find reasonably priced one person bivy bags in the kind of material you're looking for? Why not take two of them and stitch one-and-a-half of them together?


You could also be on the lookout for some lightweight surplus material around you. A "top-hat" for a car (used in winter time to keep frost from forming on the windows), a thin tarp or some cut-out from an old tent. It would be light enough to make do while you search for the optimum double bivy bag and/or raw materials in unobtanium.