Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:2877706.
We've all seen, and printed, the XYZ calibration cubes. Not only do they work great to test dimensional accuracy as well as z-wobble, ghosting and a host of other issues, but when printed in vase mode, they work great to purge filament in between color changes. However, I have found that some filament will print beautifully on the straight edges of the cube, but problems show up when printing rounded or curved surfaces.
To that end, I have made this simple, hollowed out, high resolution, 20mm sphere with a flattened bottom. The 0.8mm thick walls are meant to be printed with a 0.4mm nozzle. Printing at 40mm/sec takes just under an hour and uses about 4g of filament.