I'm having a diesel tank and a water tank made by the folks at Triple M (link below). The cost of the simple rectangular water tank was comparable to the plastic tanks commercially available. The diesel tank is a somewhat elaborate, complicated shape, designed to fit the available space, and they have no trouble doing so. They have been around awhile and do good work from everything I've heard. I have an article from Nigel Calder (hat tip to John from this board) where he says he does not think metal tanks are appropriate to use on boats.