Having not designed an original mechanical part to fit onto a commercial product, I used my calipers to measure every dimension of the original knob where it interfaces with the car - the shaft receptacle and the proper circumference lip design.  The top at this point was just a simple stand-in.  I printed this as a proof-of-concept for the basic functional design and to prove that my 3D printer (Flux 3D Delta) could give me the accuracy that this project needed.