Factory made roof racks are ridiculously expensive and the structural design doesn't look very sound. Therefore I made my own from materials found at a local scrap yard and had them galvanized locally. They seem to work well so far.
30x30x3mm square tube for the bar and 3mm plate for the end pieces. 5mm plate for the flanges that bolt to the roof. All stick welded together using Elga P48 rods. 3mm reinforced rubber mat between the flanges and the roof.