Summary

Use "fancy 3d printable cord stopper" instead. It is the same as this but with a provision for an additional circle spring and grippy teeth and is easily customizable.

Instructions

There are some parameters in the SCAD file to adjust for various dimensions and also for the part clearance, etc. The STL is the one I printed out (in ABS) and am using now. I recommend only making the spring thickness equal to your extrusion width. I tried it with multiple perimeters to increase the tension but the additional layers reduces the elasticity. I may tweak it again with multiple, unconnected spring perimeters if I get inspired.

I imagine that the springiness will not hold up well over time due to the nature of thermoplastics, so I may make another one that can be used with a standard compression spring, but I wanted to make one that was 100% 3d printable just for the challenge of it.

3d printable cord stopper - first draft