Here are the controls…
…and the ratios available:
This isn’t an effect, you’ll find it in the titles browser:
Drag onto the timeline so that it is above all the content you’d like to matte:
As the matte is on its own layer you can reposition clips under the matte using the transform control, or by changing the scale and position parameters in the inspector.
For more about how to use title generators as mattes, check out the instructions in my ‘Alex4D Dimensions’ post. In that post there are instructions for how to use Compressor and other free tools to crop off the matte from your outputted movies.
Download the effect archive for Final Cut Pro X here.
Expand the ZIP archive and add the four folders to the Motion Templates folder in the Movies folder of your home directory. Create an Alex4D folder in the Titles sub-folder:
If you don’t have Motion installed, you won’t have a Motion Templates folder in your Movies folder. Download this set of empty folders and add them to the Movies folder of your home directory.
Although the folder is called Motion Templates, you don’t need to have Motion installed on your computer for this effect to work.
Check out my other free effects and articles on my Final Cut Pro X home page.