A much simpler way is to use the FCP lower 3rds text gen. It has a solid background and you can adjust color and opacity. You can crop the size using the crop adjustments in the motion tab and even feather the edges.
If you want a more sophisticated solid bar, the free Alex4D plugin version gives more control over the solids shape and gradient.
http://alex4d.wordpress.com/2009/03/30/free-final-cut-plugin-lower-third/
And don't forget to make a donation if you like and use the plugin and keep good guys like Alex happy.
