This simple pushes onto the top of the middle spacer to cover the flight controller, protecting it should we crash-land upside down.
ping_sensor_cover from civrays on Sketchfab.
You can download the .stl or Freecad files here;
Don't have a 3D printer? If you are in the UK you cano order one from civrays via makexyz.
Please check out the other 3D parts;