Rather than use a square prop as suggested in my last post, you can use a special prop I made for this purpose "TargetAnimOff.pp2". The advantage of using this prop is that you can set its hight in any frame of the animation and it will maintain that hight throughout the animation (this is because it has the 'static' atribute set to 1). Another advantage is that you do not need to hide it if you are going to render the animation, the prop will not show in rendered images.

Copy the text in the above link and paste it into a text editor, save it to a props pallet as "TargetAnimOff.pp2".