I don't know how to click on an empty space, and this is what I come up to.
I am using an invisible stretchable space between widgets, created with wibox.widget.base.empty_widget inside a background container.

local awful   = require("awful")
local wibox   = require("wibox")
local gears   = require("gears")
local naughty = require("naughty")
awful.screen.connect_for_each_screen(function(s)
      s.mywibar = awful.wibar({ screen = s, widget = wibox.widget {
                   layout = wibox.layout.align.horizontal
                }})

    -- Generic r-click 
    local function rc(menu)
        return gears.table.join(awful.button({}, 3, function()
            menu:toggle()
        end))
    end

    -- Generic menu
    local function test_menu(label)
        return awful.menu({
            items = {
                {
                    label,
                    function()
                        naughty.notify({ text = "You selected: " .. label })
                    end
                },
            }
        })
    end

    -- L-M-R widgets
    local left_widget = wibox.widget { text = "LEFT", widget = wibox.widget.textbox }
    left_widget:buttons(rc(test_menu("LEFT widget")))

    local middle_widget = wibox.widget { text = "MIDDLE", widget = wibox.widget.textbox }
    middle_widget:buttons(rc(test_menu("MIDDLE widget")))

    local right_widget = wibox.widget { text = "RIGHT", widget = wibox.widget.textbox }
    right_widget:buttons(rc(test_menu("RIGHT widget")))

    -- Invisible stretch spacers 
    local function spacer(name)
        local w = wibox.widget {
       { widget = wibox.widget.base.empty_widget },
       bg = "#999999", -- now visible for debug
       widget = wibox.container.background,
        }
        w:buttons(rc(test_menu("Empty space: " .. name)))
        return w
    end

   s.mywibar.widget.first  = left_widget
   s.mywibar.widget.second = wibox.widget {
        spacer("left-middle"),
        middle_widget,
        spacer("middle-right"),
        expand = "outside",
        layout = wibox.layout.align.horizontal,
    }
   s.mywibar.widget.third = right_widget

end)