These crossbows spread across standard military issue cargo cover bows. (Bows not included.)

The five bars attach to your cargo bows using clamp fittings (5mm hex wrench needed) and provide more support and profile than the typical straps. 
 
Note: If your bows have turnbuckles, the "center" crossbar will not be, well, centered. It will be 1" to one side. You could remove your center turnbuckle if desired.