Vertex k8400 z-endstop extender

DylanVanAssche's [z-endstop extender](https://www.thingiverse.com/thing:1341972) was a good start, but I wanted to remove the glue from the process.

If you are reasonably sure that you can print accurately, I think this part is preferable.

To install:

1. Raise the bed to the top position.
2. Flip the printer so that is lying face down, with the rear pointed upwards.
3. Remove the four screws that hold the z-axis carriage to the bed.
4. Gently pull the z-rod away from the body of the