Doing it in steel would solve a lot of leak issues. But the cost would be up there too. If it were me I'd go with an angle iron frame and sheath it on the inside with the thinnest gauge steel your comfortable welding. It might be a good idea to construct it as segments that bolt together. After fabricating the segments have them powder coated. Would probably be cheaper and almost as durable as epoxy. During assembly apply silicon liberally to the joints before bolting them together. That way you could eventually take the thing apart if you needed to move it.