like this:
Search the rectangle (id: panelAreaBackground) in /usr/share/unity8/Panel/ and modify the line
color: callHint.visible ? theme.palette.normal.activity : theme.palette.normal.background
with:
color: callHint.visible ? theme.palette.normal.activity : "#00111111"
then modify the PanelMenu object (id: __indicators) and replace the color property with:
panelColor: indicators.fullyClosed ? panelAreaBackground.color : "#111111"
to prevent the opacity staying modified when opening the panel menus.
I made the opacity of the panel editable through a dialog popup when keeping pressed on wallpaper, but for this reason the changes are more complicated (modifying /usr/share/unity8/Stage.qml and /usr/share/unity8/Components/PanelState/PanelState.qml files).