Don't forget about the caps. But most probably the moisture is the real killer. Watertight casing is not the best idea as moisture will be trapped inside. If the box is somewhat isolated a heating element of 1W should be sufficient. Such "heating elements" are also more commonly known as "resistor"
That is connect a 100 ohm, 2W resistor in parallel to the power supply (or out the power supply into the same box) and it should be warm enough. Notice that you do not need to be to warm, just some degrees warmer than the surroundings.