I really don't know what to tell you with that design what would or wouldn't work. I think you may be a little more successful with something like 5-10mm pond liner but I have not knowledge of how to seal it in at the bottom edge.
The good think about the design is the uniqueness of the steps. Unfortunately that is the problem. The only thing I can really think of is to make a box and coat it completely. Then make each board of the steps and coat them individually, then add them, then put the tank at an angle and pour in the corners to seal it up that way so that no water gets under the steps. It is a LOT of work.
Sorry I can't be of more help. You are stepping out of the norm on this one! Good for you and I really hope it works!
Edit: The other option is to use fiberglass at least as a base and then put the epoxy on top of it. That may work...