Excellent design by sneakypoo. Thank you.
My remix fits the Vertex K8400 without modifications and without extra hardware, except for one M3x12 screw/bolt. As the original, it allows you to use several sizes of spools.
Since printers often print hole sizes too narrow, you probably need to resize the parts somewhat. Obviously, the ball bearings must fit inside the core, and the core threads must fit inside the nut threads. I resized the core part +3% in X and Y direction, and the nut part +5%, also in X and Y direction. The Z direction size changes must be the same in both parts, or the threads will bite and lock one another. I left Z sizes unchanged. These are my figures. Your printer might be better tuned than mine. Make test prints, 10 mm high or so, and check that the parts fits not too tight not too loose, before you start the full 5 hour print. I wish I had done so. The supplied stl files are not resized.
The screw keeps the core in place. A tooth pick and a small glue blob will probably also work.
Print the core part "mushroom style", the pointy end downwards, there is one small horizontal surface inside. You might get away with printing it other end up, have not tried that. As it is, it definitely needs a brim, I used a 10 mm one.