Measure the degree of squeezing a ball

Hi guys.
I'm trying to measure how much a squeeze ball is squeezed. I tried to attach a flex sensor to the surface of the ball, but the result isn't accurate enough. Do you have any ideas how easy and accurately accomplish this goal?

What mechanism is being used to squeeze it? Could that have an encoder or such added to it?

It is simply squeezed with your hand.

Try an air-filled ball and measure the increase in air pressure.

Paul