I've created an FCP X effect that adds many more aspect ratios, a portrait option, a density control and position controls:
One aspect ratio is 2.39:1 - 35mm Anamorphic:
Here is the list of aspect ratios:
The aspect ratios also work vertically.
Here's a slightly transparent matte showing the visible area for a movie designed to be viewed on an iPhone being held vertically:
For download link and install instructions, visit my blog:
alex4d.wordpress.com/2011/07/22/fcpx-widescreen-matte/
Alex