These are not made anymore so it is a case of finding a good secondhand one, or having one specially fabricated.

Some manufacturers make universal fitting models in component form to allow several differing mounting options with fully adjustable brackets. Outback do such an item, it is manufactured from aluminium extrusions, so is extremely strong, and its individual component structure allows it to fit any vehicle. This may be by drilling the roof and fitting one of their mounting rails directly to the roof, or buying the mounting brackets individually for gutter mounting.