Install box right there! Inset about 1/2" so that the knobs are flush with the front of the piano. That way they won't stick out to far and accidentally get damaged. Of course it looks better if it's back a bit as well.
AirPort Express
Dual Source Box
YouTube Video Shows dual source box mounted on piano...
Switch Out = Source 1 (MacMini)

Switch In= Source 2
(AirPort Express)
IQ Box Controller under piano.