I worked at a garage for ten years and used to see this problem all the time. We had a socket the same as the one in the pic and it worked great aslong as the locking nuts/bolts wernt done up too tight. The other way is the method mentioned above, cheap sockets and hammer them on, but make sure you hammer them on well and get a nice tight fit as you may only get one chance before they round. All this is depending on the type of locking nut of course.
If you have the standard Fiat locking nuts you may be able to borrow a spare socket of a mate or someone else with a Fiat. When I was at Fiat we kept a selection of about 5 or 6 and that was enough to remove most of the nuts we encountered.
Hope this will help, Mark.