First I cut strips of Plexiglas and used clear silicone to seal the sliding channels of the Skybox base. This removes the adjustability of it (only goes on this car which I'll have for decades) but makes the box water tight. Then I figured out where the crossbars would be positioned on the roof, where the box would be positioned on the crossbars for the rear hatch to open all the way without hitting. Then I marked the position on the base of the Skybox and the crossbars