A very cheap and rather effective method is to use the conductive foam (the black stuff) that IC's come packed in.
If you measure the resistance of the foam you will find the resistance reduces as you compress the foam, so it can be used as a variable resistor. (or compression resistor)
The mechanics to control the foam compression would need a little working out, but it is a rather consistant result you get from the foam and im sure it would be way better than within 20%.
Doing an ADC reading on the foam with a micro would give a very reasonable result.
If this is for a commercial application then this method might not be the best but for a simple quick small use application i think it will work ok.
I know of a throttle position sensor made this way for a addon circuit that has worked for several years without fail.
Mount the foam between 2 bits of copper clad PCB board so wires can be soldered to the 2 boards.
A quick test i just done with the multimeter probes gave 30K to 10K full scale and i think a 1 inch square would take more than 1 kg to compress it.
Food for thought.
Pete.