Check out your local sporting goods store and look for 'bear scare', i.e. pepper spray. They come in varying sizes and you can get nylon holsters for the cans which can be strapped/taped to the top tube for easy access.
One consideration though, if you ever have to use the stuff, is to be aware of the wind direction before deploying the trigger. It would not only incapicitate you should it blow back in your face, but would surely enrage your attacker to possibly do something more to you in your defenseless state. A slight breeze is all it would take to blow the spray off target. Having said that, you would hope just pointing a can of that stuff at somebody would be sufficient reason for them to back off and leave you alone.