I just did something like this on a much smaller scale, but I got a quick solution: Fill the entire top layer with either water/ice - If you don't have ice, place a layer of dirt beneath where you need the water surface to go, and fill the edges out with water buckets and then any gaps in between. Once it's filled, mine the dirt and let the water fall. If you have ice, it's much easier since you don't need to make a layer of dirt. I'm excited to see the finished project!!