Do you have a screen print screen with tray? Not sure what paint will stick to plastic or waterproof fabric that you can screen. Maybe stencil is the way to go.
Have you tried stenciling it? maybe mix in some fabric medium to make the paint more flexible?
You could stenciled pictures on it
I would recommend a stencil or using painter's tape for a geometric pattern. Use fabric paint.
Mix acrylic paint mixed with fabric medium so it will flex.