This is a remix of Robert Marchese's design: https://www.thingiverse.com/thing:3370200

The new features that can be customized are as follows: v1.1 - 2019-06-15 (included Robert Marchese's suggestions, thank you):

• fixed arguments warnings with rotate()
• added descriptions to parameters shown in the Customizer GUI
• added parameter to pull the shell in the Z axis
• added parameter to control angle used to add rings to the shell
• added parameters to orient shell in XYZ axis

v1.0 - 2019-06-10:

• the shape of the rings used to create the shell can be cylinder (original design shape), multiple sided polygon, or sphere.
• the number of rings used to create the shell (greater that 40 rings turn more that 360 degrees).
• rings can be scaled in x axis to create elliptical shape, this scaling is specified for the first ring (smallest) and the last ring (largest). The scaling for the rings between the fist and last ring is a linear calculation.
• the number of sides for polygon shaped rings
• polygon rings can be rotated/spun relative to the previous ring and the amount of relative spin is specified as an angular degree of movement.

The "shpere" ring shape is a work in progress. Shells created using this ring shape are probably not printable.

When "Poloygon" ring shapes are selected the customizer is slow to generate the preview image. Limit the number of rings as you change the other parameters, make selection of the number of rings the last parameter you change.

I run openscad on my computer (2GHz i7 CPU with 12 GB of memory) with this model. A shell using the "poloygon" ring shape with 60 rings takes ~12 sec. to generate the preview image and ~4 min. to render to save as an STL file.