The mechanics will be using a servo with a pushrod. .
If you do not have a servo with a single sided arm cut off one side of the double arm. I scored the base of one servo arm with the cardboard knife and snapped it off.
Cut a 3/4 inch strip of cardboard. Make sure the inner corrugation (ridges) are aligned the short way. You are going to thread the paper clip through the corrugation.
Take a standard paper clip and bend it as shown. I unfolded this one and used its existing bends. Thread the hook side though the servo arm.
Line up and place the servo on the stationary side if the door panel. Place it next the stationary side of the hinge. Use hot glue or double sided tape to secure it. Your done! Make sure you disconnect your servo are before you program it for the first time. You need to learn its position!
Here is a close-up of the pushrod and servo.
It’s up to you to decide what happens to the ball when it gets inside 🙂