the door handle is held in place by a single nut on a threaded part of the outer handle with a sort of pressed cup that goes over this threaded part and pulls the outer handle against the door skin when the nut is tightened... if you tighten the nut up too much then the cup squashes and no matter how much you tighten it will not make things tighter. Some people put an extra nut or washer believing this will help it will not as the up then touches the door handle..
If you take the handle off it will become obvious, I have looked at making an alternative using a very short peice of tubing (thicker) squashed to make roughly the same shape, and then puttting a thicker peice of flat steel on top that will not crush when tightened up. Hope you understand that.
On early 500s (asssume Giardiniera's are the same) there is surprisingly no gasket between the handle and the door skin but it is possible to get a moulded gasket that will fit think they are from Multiplas or 600s