Hi Bob,
Here's what I do...
Create a graphic, (Photoshop, Motion, etc.) up to the size of your video clip.
Import into FCP.
Place it on the timeline over your video at the correct length.
Load the graphic clip into the Viewer, select Motion tab.
Crop the graphic to the size of a lower third. (Adding feathering usually makes it look better to me).
Create desired text in FCP (or Livetype or Photoshop if you need something fancy.)
Import the text file into FCP.
Superimpose it over your lower third, position it correctly. (Verify that FCP doesn't automatically apply distortion or scaling to your graphics or text when you place them into the timeline).
Tweak your crop, scale, etc. settings within FCP, and/or right-click the graphics/text in the timeline and choose "Open in Editor" to open in external editor.
Also, a dissolve transition usually works nicely at the beginning and end of a lower third and its overlaying text.
I have found that many types of files (video, animations, stills) work as the basis for a lower third, and there must be thousands of options for colorizing, scaling, adding filters, animating, etc. once you have the basics mastered.
There may be other ways to do it, but that's my process...
Hope it helps, let me know if you have questions,
jeff
Dual G5 2.0, Dual 23" Cinema Displays, 2.5 GB RAM Mac OS X (10.4.7) FCP 5.1.1, DVDSP 4.0.3, HVR-Z1U, HVR-HC1, HVR-M10U