I was looking for a luggage rack for my B a couple of years ago, and held out until an original one became available. The main difference between the original and the generic one posted above is that the four 'feet' are bigger on the FIAT one and are located so that they bear on both the boot edge and the rear wings. If memory serves me right, the B's boot went through various revisions of steel guage in order to combat dings/ distortions better. The further the foot is from the anchors, the greater the risk of deformation in my mind, and the original incorporates the anchor point within the bearing foot... a neat solution.
However, finding one is tricky now, and prices are rather steep... So all I can add is that the general rack shown above is pretty close in aesthetics and compliments the B, but be warey of over tightening the fittings and don't load it up with a 20kg suitcase.
Worth checking if the general rack has little slots on the underside of the rails to feed straps through too... v useful when trying to secure luggage.
