We've shown you a couple of ways to fix your posture and why it's so important to do so. However, if they aren't working out for you, this posture sensor might do the job.
The folks at Coretech Robotics put together this project. It does take a little skill to build, but it's nothing too daunting. They used an SR-HC04 sensor, because it's cheap and reasonably precise. An Attiny85 microcontroller, small Piezo speaker and a power supply round out the package. Both the sensor and the Attiny85 are programmed using Arduino.