Force-Sensitive Resistor Break When Pressure is Constantly Applied
Perhaps there are better sensors to try, e.g. a strain gauge. An entire electronic scale with its strain gauge sensor can be found for ~US$10. Strain gauges have much lower drift than piezoresistors. Also look at capacitive load cells.

On the other hand, you could experiment with carbon granule piezoresistors, as used in carbon microphones, or a carbon/foam sensor. However, these also have high drift.