Physical a "metal piece of art" would be the same as metal in the wall switch frame i made my switch from.
Except that the switch frame would look better in the bathroom then "metal art" 
Physical i think a box with some slots at bottom and top for airflow would be best.
I saw now that they have surface mount housings from my switch line: http://www.lichtchef.de/images/jung2/AS581ASW.jpg
But i think this will be overkill for the small DS18B20.
