Plasma 6 applet that enables/disables transparency for panels. This is a fork of the original Plasma 5 applet by psifidotos.
demonstration
tranparent panel when active
solid panel when inactive
Add the button widget to your panel and activate it. Your panel will become completely transparent; just exit the panel configuration mode to hide the button.
This is a QML applet and as such it can be easily installed from Discover or the "Get New" applet in System Settings. Alternatively, you can execute kpackagetool6 -i . in the root directory of the applet.