We started thinking about the sensor mechanical fabrication today. It needs to be robust, moisture resistant, corrosion resistant, able to be pushed / twisted by a human into possibly compacted ground, and maybe even driven over. The shaft at this stage may be up to 20mm aluminium tube, with 3D printed bushes at each end and threading to accept a possibly ABS 3D printed structure housing the electronics and solar panel. The shaft may even have a auger incorporated

The image shows also our thoughts about how the DS18B20 sensor end be in ground contact, perhaps via a fabricated tapered aluminium block and heatsink grease.