As long as enclosure bottom is waterproof, a pool is easy to design and a small filter can help keep it clean and healthy. Adding waterfalls etc. can complicate plan to headache level. If you are resealing the silicone; need to remove all the old one and clean with acetone (wear breathing protection) before re-glueing, since silicone does not stick to old silicone well.

With that 36 in height you have many design options including both a floor and a canopy design. IRT materials; myself would concentrate on lightweight items due to enclosure size. For example replacing a clay ball layer with sintered glass aquarium media (i.e. Seachem Matrix) or with a dead space can save you some serious weight in such large area .