Water Block and a static shape

Is there a way to clip a square water block based on a static shape? What i am trying to do is place a pool of water in an elevated cistern, the cistern is non square so to fill the cistern with a torque water block I end up with the corners sticking out of the shape, is there a way to use the water block for this purpose or am i going to have to suck it up and create a plane of that shape and apply a water shader to it?