A few days ago the company CoreMelt, developer of video editing and compositing plugins, announced its free product SliceX Free for Final Cut Pro X.
The great thing about SliceX is that it is powered by the renowned mocha technology to achieve very good results in mask tracking. Thanks to this plugin you will be able to do tracking applied to rectangular and circular masks.
In addition to the free version, SliceX has a paid version ($99) with additional features such as drawing and tracking masks with full perspective transformation and the ability to add keyframes.