If you get one from Plasticwatertanks I would get a flat tank and by flat I mean about 18" high from front to back of the van,it will enable you to keep nearly all your van for storing other stuff. If you stick a sheet of ply over the top you'll never know it's there,it won't need bolting or strapping it it's measured correctly it can sit between the wheel arches and they act as anchors.