Here is a project you can make entirely on a 3D printer. Minimal CAD experience is required to create your own design. If you own a 3D printer great, if not use 3Dhubs.com to outsource the job. Below you will find the instructions.
Step 1: First Sketch Out the Design Relative to Your Hand
By creating a sketch of the design on paper you can compare it to your hand size for best ergonomics. When creating a self defense tool it is good to have a comfortable base that will be stabilized by the palm and a pointy part for gouging in the event of an emergency. With those factors in mind you can get creative with your design or use the design I created.
Step 2: Use CAD Software to Create the 3D Print File
I chose to work with Autodesk Fusion 360 which makes a design like this very easy to create. In Fusion 360 you can start with a 2D sketch and then simply extrude the design to give it shape and then add a few fillets for a comfortable fit.
Any CAD software that you feel comfortable with will work just be sure the dimensions you choose are a good fit for your hand.
If you would like to use the design file I created, here is the .STL file. SelfDefenseKeychain
Step 3: Print You 3D File on a 3D Printer or Service
If you own a 3D printer then fire it up and start printing. I chose to do mine in ECO friendly PLA plastic which is also fairly strong. A 40% fill volume seemed to work well for this print.
If you want to print something but you do not own a 3D printer then consider using 3Dhubs.com where they can match you up with available printers in your area who will print your design for you and you can either have it shipped to you or pick it up. 3Dhubs is priced very reasonably and is a great resource when you are getting started making.
Step 4: Defend Yourself!
Hopefully you never need to use this keychain for self defense, but if you ever find yourself in such a scenario you will probably be glad you had it.
Walk softly but carry a big keychain....