Designed this soap holder using the idea of a loosely folded sheet of paper, which is also closely related to the shape of a meandering river. Design was completed using SciPy integration and CadQuery based on the equations developed by Leopold and Langbein. 

The model is slightly springy but sits flat. The CadQuery source code for this model is included, so please remix as you wish!

I printed the pictured version using spiral vase-mode with a 0.6mm extrusion width, and it printed in only about 2.5 hours! I recommend you print with the included orientation in the STL with the flat side of the curves touching the buildplate.

See here for more information about these curves:
https://mathcurve.com/courbes2d.gb/giration/motifs.shtml