I twist together two lengths of kevlar fishing line that's 0.4mm diameter and rated for 29kg, so combined roughly 0.8mm diameter and ~58kg pull. This is about the sweet spot imo in terms of thickness, slim enough to fit in small routing holes on mainspring housings and thick enough to get a good sized knot when you tie it. It's also pretty close to the thickness of old sinew drawbands I've replaced. The rated strength is definitely overkill but better over than under. In practise a drawband shouldn't experience more than 750g-1,5kg of pull under normal use.