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.