Don't know how cotton would work. B-50 dacron or linen cord/thread work good in the store-bought department. If you wanna make prmitive strings, sinew, gut, or rawhide are probably the best bet. With rawhide, cut a piece in a spiral to get a long thong about 1/8"-1/4" or so wide. Try to cut it as uniform in width as you can. You can test it with a scale to find its breaking strength and figure out how many strands you need. Wet it, twist it up tight, and hang a heavy weight from it until it dries.