Here's a simple idea that serves a dual purpose...if someone attempts to enter your room while you are inside, the alarm alerts you of intrusion and probably scares them off.  It's compact and easy-to-use.  When in your hotel or dorm room, close and lock your door and slip the hang cord over the inside door handle. Then hang on door handle inside your room and activate the alarm. If the door is opened, the vibration activates a loud alarm sound.