Plasma now supports different looks for panels depending on the screen edge where the panel is located. You can find more details about how to make this kind of themes at KDE’s Techbase. Basically if you want to create a custom look for a left panel, just create all background elements for it and add prefix ‘west-‘ to the name of every element. (east for right, north for top and south for bottom panel).
If you don’t want to name all the elements yourself, you could always use the Slim Glow theme as a base template since it uses this new feature of Plasma.
Reminder: Start making the Plasma themes, there is a contest going on!