Look on the big auction site for keychain self defense tool give you some ideas maybe kubaton,Capos,Munios or something along those lines,I do lots of walking and encounter aggressive people/dogs every once in a while and its an awful feeling to not have any personal protection on you ,so I am always looking for self protection tools thats easy to carry/hide that wont freak people out but will help against possible attack a customized version would be interesting for sure.
I know its not a knife, but sometimes it keeps the creativity flowing to change things up a bit,heck i've put together a few Capos from some scrap oak and paracord and they look great used my knife to shape it the way I want to,when your walking alone something is better than not having anything on you.