I use a large contractors plastic one that came from Home Depot. It sits at the tail gate with a short bungee on either side from the rear interior tie down to the tub. It never moves and I lock it when I need to.
Works great when going on trips (fishing, camping, hiking, etc.) Holds my middle sized cooler, luggage, picnic items and at all other times my tonneau cover, tie downs, straps, etc.
I've also found that the hieght is perfect for running 2x4's through the back window and sitting them on the tail gate and tub, then tieing down the 2x4's between the tub and the tail gate. This keeps the 2x4's off the rear seats.