hmm... our cistern is flat in the bottom (like picture on the left) but raised at the lid (as in picture on the right).
this "floatie" that regulates water level is nearly at the very top. there is only two of us in the house and we use very little water: no pool, no garden sprinkles, only daily usage. there is no need to use the full capacity of the cistern, i.e. fill it to the brim.
oh, end edges are sealed with cement. it's all nicely evened and plastered all around. some people use pool paint, i think. in a way a cistern IS like a pool. you have to assure there is no filtration, keep water clean (we add a chloro pill every now and then) and wash the whole thing occasionally (when we moved we emptied the cistern and had it cleaned on the inside).