How about a tire knocker?
They're cheap, made of hickory, with a big metal stud in the end for striking weight. Used for checking the pressure of truck tires, but it oughta work on bike tires, too. Right?
Or just fill an old, sturdy frame pump with fishing weights.

Self defense classes would be the most effective. A little close quarters training will render a club useless.
I used to work with a Wing Tsun instructor; his philosophy was this: Run away. If you can't outrun the attacker, break his knees and run away. If the guy has a knife, run faster. If the guy has a gun, run away, but in zigzags.