The numbers work better with increasing size, because volume is the cube of the dimensions. Also, you can build a pond on a hill with some excavators much more efficiently per unit volume than an enclosed plastic container.
A 1m cube weighs 1,000kg and has about 20Wh. A 10m cube weighs 1,000,000 kg and has about 20,000 Wh. A 50m deep, 1 km square reservoir has 1,000,000 Wh. And that's still assuming that it's 7m above the turbine!
A 1m cube weighs 1,000kg and has about 20Wh. A 10m cube weighs 1,000,000 kg and has about 20,000 Wh. A 50m deep, 1 km square reservoir has 1,000,000 Wh. And that's still assuming that it's 7m above the turbine!