I really liked the idea of the Slinky design by Desktop_Makes, but I had a hard time getting it to print cleanly. One thing I noticed is that the internal face of the Slinky was turning into a polygon due to the fact that the perimeters on the layers just above the gaps between the coils were "cutting the corner" due to not having anything to stick to. That inspired me to remix the design to make the inner wall a polygon that corresponds with the layers generated by the slicer.

This was also a fun project because I wanted to see how it could be designed on Onshape as opposed to Fusion 360, and it turns out that it was simpler in Onshape.

This design assumes that the slicer is configured for 0.2mm layer height, including the first layer. You can verify that the layer height is configured correctly by checking to see that the layer edges are aligned with the vertices of the internal polygon as shown in the attached screenshot. I used Slic3r, so it's possible that other slicers won't handle the coil in the same way.

The gap between the coils is 0.4 mm, so it took a bit of work to get them separated once the print was done, but I was able to get it done without breaking my new toy.

Enjoy!