I have the 90 gallon auxiliary tank with a gravity flow setup. Bought it at Northern Tool and my Ford dealer did the install. Its been perfect, except if I don't shut the fuel flow off at the tank, it will leak some when the truck sits for a long time. We think its coming out of the vent lines of the stock tank.
Probably something we'll fix over the winter.
Mine has the tool box on top. It locks at the cap and the box lid locks. Its been great. 125 gallons to fill up when I'm on empty.