If you mount a steel step stool on your van, you probably hang it from a hook or clip at the top.
This allows it to swing, so it'll be noisy when you're driving. If the stool is on a door, it might swing and bang when you open the door.
So mount some magnets near the bottom of the step stool. The top of it will be secured, the bottom will stick to the door, and it won't move anymore!
