Not forgetting that we need to put an enclosure *somewhere*, and that simplicity and protection are key design factors, it will be important to have more than one option.  There now exists a cover and extender for any combination of externally powered "innards", e.g. Arduino alone, power supply alone, or both together.  The Arduino connections come out through the case, and the main power supply line can be drilled and custom fit.

There's also an option for putting everything into one enclosure and keeping all but the most basic of wiring internal - a custom hole can be drilled for whatever power and data lines need to be run into the enclosure, then it can be sealed shut against the elements.