fitting a roof top box

Hi there,

I am a new motorhomer and also a new member and am seeking a bit of advice. We have just bought a 1989 Fiat Ducatto Autostratus and would like to fit a top box for extra storage. The roof has a rail around the back but I have been informed that no bars are in circulation that will fit this rail. I have looked at the Fiamma roof bars and it appears that these attatch straight on to the roof ? Has any one fitted any on here ? what time of fitting screw is needed? Any advice gratefully recieved

Ann Marie