There are posture sensors, like a iot cushion ( http://darma.co ) or the MIT's modular sensor stripe, and stick it to your back.
But these things are quite inpractible for a student who is working on different places and different occations, but wants a strait back when sitting on his tiny macbook air.
A friend of mine gave me a tip that the lower back (pelvis) is quite important for that, so my construction considers that.
The algorithm will be simple: When the average posture of 5 minutes is different from the calibrated "ideal" posture, then the critical position will vibrate and for demonstration flash up.
This way, it reminds constantly on the bad posture. (I hope at some point, I don't need this anymore)
Further approaches will be to add data-output by UART and IR-LED or even BLE (HM10) but this will be done on MK2.