There are two parts here: A TPU “spring” I designed to add reseistance to the shifter and return the lever to neutral and a modification of the original shift gate. Now, it feels and works more like a real shifter. Print all of the other original models parts as recomended. 

You print the spring from TPU with tree support, but snip the support off rather than attempt to pull it off; It doesnt matter that its clean or not. I used Sunlu 95A from Amazon. Don't feed this from AMS. Use a Y-splitter. My P1S prints 95A without issues, but YMMV. You will also get some stringing between the arms of the spring which has no effect. I find that the top edges of the curves of the spring arms are also not super clean, but it doesn't affect it's functioning. You can experiment with different print settings. I would recommend gluing the base of the spring to the base of the shifter cup from the original model to prevent it riding up in the cup and loosening when shifting. The threaded end of the shift lever itself can be pushed through the hole in the spring, or turned and screwed through the TPU. Its slighly smaller than the threads and same 10mm diameter as the lever to hold it snuggly. Once assembed and the shifter knob and lever is aligned, you can add a dab of glue to lever where it penetrates the spring, although this isn't necessary. I may design some further mods of the cup and spring to eliminate the need for gluing when I'm bored. 

I've modified the orginal design's shift gate to retain the selected gear. Print this as you would the original. This will wear overtime and become smoother, but manage expectations. This is plastic. The nubs that keep the lever in gear are not at the top surface to allow people to print it with a clean looking top in a different color than the nubs for appearance sake. This replaces the component coperchio.stl from the original model. I would also recomend gluing this to the cup. I also may mod this to be a removeable TPU piece at some point.