Cropping frame in quicktime pro

I have read several posts about cropping the size of video in quicktime pro. While the technique of adding a black mask works, as far as I can tell it only works if you are trying to crop out a central region of the movie. I am trying to crop away the right half of the movie leaving the left half. The mask shows up in the center of the field and I can't see a way to shift it to the left so I can remove the right hand portion.