Electronic devices in outdoor use
Add a low power, incandescent lamp or heater to the box to ensure that the components remain dry.  The heat will prevent temperature fluctuations that causes condensation.  In most electrical junction boxes the heaters run at about 40 celsius and have no temperature control.

Putting the power supply inside the box will warm it a little and also reduce the possibility of condensation.  You might be able to get away with no heating element.