This is the skid plate/support for the dumpster-find 19-gallon F350 auxiliary tank, which I coated with a POR-15-type product from KBS:
I was incredibly lucky to find this tank, because it fits the space on the rear of the truck, it will allow a usable range, and it was free. It, and its ancillaries, are worth their weight to me.
The end result on the tank support, air tank, spare carrier, and other parts.