A key part of this aquarium from the start was the inclusion of a bog type garden area above and behind the tank. This is a first attempt at building a support for a shelf. Actually, it wasn't the first attempt, the first attempt was a 2x4 frame that used lumber that was far from straight, thus the resulting frame was not flat. This was an attempt to make due with the wood despite its deformities.
Feeling a bit impatient, I made use of some particle board furniture pieces I had laying around and put something together. It is supported by the wall itself though and that is something I wanted to avoid. Depending on what the structure on top is built with, it may need to be rebuilt.
The plastic shown is 40mil plastic sheeting that is sold for lining shower floors before laying tile I think. It is pretty thick and therefore very durable looking. It's not cheap however, almost $6 per linear foot. But the goal was to establish a means where no matter what happened up top, the water would always end up back in the tank.