Read on to see how we made ours:
Using the Artiphon app (available for free in the Apple App store), select the Fretless mode and the strum method.
Choose Bass Guitar as your tuning.
Select Bass as your sound.
Pluck the bridge with one hand and slide on the fingerboard with the other to play the INSTRUMENT 1 like an upright bass. Try switching the sound to create an entirely new effect while still strumming and plucking a fretless instrument (think: Fretless Piano!).