Only plans I've ever seen was an article in FAMA several decades ago. It shouldn't be too hard to draw up your own. Reinforced plywood boxes are easy enough to build. First one I saw used several coats of marine grade paint but there's other products now that can be used. What precisely do you have questions about?
I have decided to go ahead and start a build thread for a massive undertaking that originally started a couple years ago and is now really getting going with the exciting part. First let me say like others I am not a carpenter by trade but I think I have some really good building skills...