Non-aero Campagnolo brake levers

What is the best way to attach these to the handlebars? The brake levers I previously had attached could be loosened/tightened with the use of an allen key, but the Campy levers I have seem to be loosened/tightened with a nut. Is there a specific tool I can use that will fit, as even with the lever pressed, it is quite a narrow space.