The [http://www.korg.com/uk/products/computergear/nanokontrol2 Korg nanoKONTROL2] is a very affordable small Midi controller. At a price point similar to the Raspberry Pi, this makes it a good fit if you are looking to add console controls for to Box Of Stops.
It has a large number of buttons that can be assigned to any [[Stop|stop]], [[Coupler|coupler]] or [[Combination|combination]] in Box Of Stops, and enough sliders to act as volume controls for all the [[Bank|banks]] and the [[Master volume|master volume]]. The compact format is also well suited to positioning close to your hands while you're playing.