So OK. Cut a strip of the blue masking tape in order to stick it to the one of sides of the conopy.Be careful and won't berak the glass. Then with the sharp pencil try to outline the inner shape of the cockpit glasses.In the pic below I have marked with the yellow line you should go with to draw the shape of the glass. Push the pencil slightly holding the conpy with left hand and fingers. When you got the inner shape of the glass unstick the tape and cut the shape out with scissors.