Use a solid color background. FCP can generate this no problem. Click the generators button(the button with the A) in the view window. I believe it is either in Matte or Other. I'm not in front of FCP to check.
Place that color BG on a new layer, and use the mask shape filter to create the line. Adjust the height and width of the mask to fit your needs.
Or you can just create a line in a graphics program with the correct resolution, dimensions, and an alpha channel to achieve the same thing.