I've used sailing whipping twine to hand-sew my old tramp, holding up well. presumably medium, needs a big needle but not a traditional sailcloth needle

http://marinestore.co.uk/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=whipping-twine

it's waxed so is easier to grasp and tighten than normal synthetics of the same size which are very slippery