Based on my researches, it depends on what you're looking for. For example, if you're looking for a better performance in small-scale ponds, inclined walls can help you a lot.
Please describe the purpose of the project. I am an energy expert and would like to know what you are going to us this for. Is it to create a power systems or are you doing reflective system or just thermal? Can you elaborate on the systems demand?
In flat land of low cost rectangular shape is chosen for efficiency of removing fill, unless the pond is filled from a central well in which case the shape is circular.
In land that is high cost and not flat the shape conforms to the topography and cost advantage.
We have built a solar pond with cylindrical shape for demonstration and teaching purposes in Costa Rica with good results for calorific energy storage, you can see some data and pictures, although the original publication is in spanish, you may get some ideas.
Solar ponds for power generation will require rectangular shape (surface) and pyramidal or cylindrical (bottom).
Shalom
Conference Paper Estudio de un Estanque Solar para Almacenar Energía Calórica...