I think you are better of printing a version of the MyHalf by Kimmo, that has basically the same functionality. But added the option. (original costs + assembling &programming - the costs of the casing and breadboard)

The PCB is finalized and ordered a shitload of them



Also, hooked about 3 euros of parts to my Jolla yesterday (5V step up and stereo amplifier) to power two full sized speakers without external power source. VIDEO
Important question: how do you determine how powerful a phone is? Processing speed? Pixels? Or amps and dB?