Before I sew the real thing I thought I should test a few different cords to see which worked best. The top cording is cotton crochet string. The middle is hemp twine. The bottom is s mystery fiber pulled from bias piping.
The cotton string is a consistent width, stays twisted so it was easy to work with, and added heft without being too stiff. The twine varied too much in width making it difficult to sew a straight line against, and even though it was thinner than the string the corded section was much stiffer. The mystery yarn separated into threads and kept flattening out, making it hard to sew against.
The cotton string wins.