Using an old cigar container, a drill and some hardware, this project is a very easy one to get started on. Many inside doors are actually hollow on the interior, making it easy to drill a small-ish hole and secure a container hidden inside the door.
Although this won't fool any law enforcement officer worth their salt, a common thief wouldn't think to look on top of a door for hidden items.
The trick is to use a hole which is counterbored, allowing the container to sit flush with the top of the door. When it comes time to remove the container, a magnet will create the force for lifting it out of its secret location.