If you have checked online for belly water tanks, you know they are not cheap. Here is a great idea from 4WD Action magazine, issue 159. An Aussie had scanned the two pages and posted them online. Apparently it is just a section of large diameter PVC cut to fit the length available outside the rails, capped and fitted with hose fittings. A galvanized sheet metal box is fabbed up to cradle and protect it.