Rain sensor

thingiverse

Here's what you can do: Buy the sensor that can be bought using Amazon or Banggood, print its base to fit your fence column, then print its cover. Inside the cover is where you'll find the frame for assembling the sensor's PCB - use silicone to make it waterproof. Glue the sensor's PCB in place, connect all wires and glue the electronic PCB too - also using silicone this time. Once done, cover the entire pcb with more silicone. Next, put the wires through a hole in the base and assemble both parts. It fits perfectly in your case, just apply a little force and it works great.