Nine-month ago, I have made the longest 3D printed flexible shaft in the world. At that time I just start learning how to design a 3D model. I have little knowledge about the mechanical and engineering field. Even the idea and concept are correct, the amateur moldering method makes the construction difficult and complex. Now I design this new version even I am still learning. I will use some cheap commercial goods to make it. It is cheap, easy, and practical. Using a Teflon tube with a stainless steel wire, you can even make it as long as you wish. I also upload the f3d file. If you change the ball bearing to a new type it will be more stable.
​Hope it will give you some help.

https://youtu.be/IreuYDumPa0