Soil sensor recommendations for Express

Since the Express does not have integrated soil moisture or temp sensors does anyone have a suggestion for rugged waterproof sensors that they have used that have provided consistent and reliable data? Lots of info out there online but I wanted to see if anyone here had suggestions as well. We are using a mobile raised bed for our Express so a solid understanding of our soil moisture is critical since drainage is minimal and we don’t want to drown our plants.

Thanks!
Joey