Summary

My son wanted to create a "turtle bot" so I created this shell in OpenScad. But we realized that in order to be big enough to enclose an Arduino Uno it would take 9 hours to print, so I never printed it.
The included STL is about 56mm wide, but with the OpenScad model or ThingiVerse Customizer you can set it to whatever you want. There is a single variable, shell_radius, which is roughly the length from the center out to the outer shell wall along the smallest axis. (It's 1.5x longer than it is wide)
I've never tried to print it. It will probably need supports in the center.
Revised: Simplified OpenScad file and commented it, because it illustrates a couple techniques for making "pancake" shapes.