A short length (18 - 24") of paracord with a knot at both ends works well also. If your country's laws and police aren't too bad on that sort of thing, you may consider adding a "monkey fist" knot at one end.
Either way, paracord makes a great flexible weapon that is useful for blocks, chokes, and holds. The monkey fist will add striking ability for soft targets. Particularly eyes, face, throat, and genitals.
A great idea! If you can't carry such a device, you can quickly improvise one from a suitable length of cord or a bandanna, a large safety pin, and a suitably sized lead sinker. The advantage of this is that you can make and discard it.