This very simple controller changes a resistor while pinching.
In an analog synth you can connect this resistor in a voltage control path or an audio path to modulate the amount of signal. Youtube movie demonstrating this:
https://youtu.be/prNeOhIGhu4

Needs just one (30 cent) Light Dependant Resistor (LDR), no other electronics or a supply.
You do need some ambient light for it to work because that is wat gets on the LDR increasingly when you pinch.
An alternative circuit version that generates a variable voltage at the output is also provided.

This project just shows the concept.
Mounting one or more of these controllers on a box or plate equiped with connectors creates a handy and cheap
controller module.

Some construction tips:
-To make place for the LDR inside the clothes pin you should remove a bit of wood using a wood-file.
-To lower the spring force for easier pinching you could stretch the spring for a slight deformation.