When it comes to handling muggers and/or aggressive critters, you might try this pepper spray
, which is designed to attach to your top tube by velcro. Props to Xela for finding and posting it in the self-defense
I bought one and have been quite happy with it in terms of ease-of-attachment and release. I have fortunately had no need to test its performance in a self-defense situation, so I can't speak to that. I like having it out of the way but immediately to-hand. It's stayed in place for several hundred miles, including a few sections of vision-blurring dirt road washboards.