Start with a half-gallon container of any kind. A half-gallon of orange juice or milk in a rectangular prism shape are ideal for fitting in the cistern without disrupting important mechanisms.
You might have heard of using a brick or stone as your displacer in the past, but I would recommend against it. Using brick or stone could (eventually) create a build-up of sediment in your tank and pipes, and that would be very costly to fix.
