How to mask everything except eyes? I have a video containing a single person facing the camera. I would like to black-out everything in this video except for a small rectangular box around the person's eyes. I'm sure there are many approaches that might work for this, but I'm having trouble figuring out where to even begin. Preferably it can be done using free software for OS X.

Edit: To clarify, the person's head moves around significantly and unpredictably, so I'm asking how to do this with some sort of motion tracking.
Davinci Resolve lite is free, and it contains an excellent motion tracker.  By combining the motion tracker with a power window, and adding a color correction (to darken everything outside of the power window), you should be able to achieve the results you're after very quickly.  For tutorials on Davinci Resolve, search youtube for Alexis Van Hurkman.  He literally wrote the book on it (the instruction manual), and provides a generous amount of tutorials online for free.  Of course, he also has a complete course for sale through ripple training.  You can download Resolve lite here.