Thanks both.
Yeah, if possible, I will put the mains and Arduino in a smaller box inside the bigger one.
Then all that goes in to the bigger box is the 12v DC wires.
The mains is contained in the smaller box with the SMPS and arduino.
I'm "down under". Solar pannels are a bit too much cost for the use. It is already costing me $200 and that is only to water 5 potted plants on my balcony. $)
Yeah, I will put a hole on the underside to allow any water which accumulates to escape.
Time for more thinking. 