Kenn, what about building the tank out of aluminum and then coating the interior with some type of protective coating, This is pretty much what they do with aluminum drink cans. Would think that an epoxy or perhaps a corrosion coating such as NO-OX-ID (
NO-OX-ID rust preventatives & conversion coating prevent corrosion) could be applied to the interior thru a removable hatch in the top or poured in and then roll the tank around to coat it and drain out the excess. not sure how that would compare cost wise or durability in the elements compared to a plywood tank. I would be afraid of moisture working its way into the plywood from inside or outside and causing delamination or rot. Then again I am way to type A and tend to over think everything lol.