Hi David,
I think what I did was to take an image of any standard 5U tall module that is the same number of units/hp wide as the CP image I wanted to create and then placed the perfectly trimmed CP image over the top of it. I then selected everything below the CP image and replaced it with the solid color. The hardest part is just proportionally resizing one or the other image so that they are exactly the same width.
One sad thing to note, not all CP panels are the same height! :-/ Details are in the thread below, basically COTK CP modules are a little taller then how everyone else makes their CP modules.
https://www.muffwiggler.com/forum/viewtopic.php?t=120103
But if you use the method I described, you wont have to worry about the height measurements since the U/hp widths are consistent.