I got a pack of 4 drapper ratchet straps for £15. Use them for tying the quads down and held no problem.
I've no idea what your carrying but I have a big dry bag, it was only a cheap one, but I stuff what I can in that, then ratchet it down, keeps things dry as well.
I have always be told that should never use rope, either the knot will loosen or get so tight it's hard to undo the other end.