I would contact an aquarium company - someplace that builds customs. Or Custom Cages. See if they can help you with planning or ideas. I built a smaller scale aquarium/terrarium combination for a red ear turtle when I was a kid, and got a lot of great advice from a local glass and window company who does custom work (we live in a small town so not many options, especially since I was like fourteen lol). They were at least able to tell me what I could and couldn't do as far as sealants and such.

The weight of water and soil would be the most concerning this for me, especially since you're going up instead of out.