Poly rope? That's what the fibers are made of, right?
It's a bit tricky, I guess. Need something that isn't going to stretch a lot - really don't need it coming slightly loose and banging around for half the trip - but at the same time, it can't be brittle.
Get plenty of rope, and maybe in two or three long pieces. Then we can tie it down several times and give it some extra stability.
I like Home Depot ...