I have completed the device by now. Mounted the case to the temperature lance by a aluminum bracket and stainless hose clamps. It may not be the prettiest setup but it's sturdy and it works. I did leave it sticked into the soil outsie for a few days now. Night temperatures below 0°C and some nice test-rain: no signs of moisture inside.

I left a bit of extra cable because who knows....

Status: finished