I have this same bar with a couple of milk crates and it works perfect. I have a bed cover so the groceries stay at arms reach and dry....:smileup:. for about 10!!!!!! its awesome....I have a cargo bar as in the link above. You can get them at HarborFreight cheap. (with the 25% coupon)
But I also have a DualLiner bed liner. The rubber mat in the bottom keeps things from moving around... and then I just have a bungee across the 2 rear tie points... and that keeps grocery bags from moving too far from the tailgate.