Having been down this road before, you are going to want to design a lid to keep your heat loss and evaporation to a minimum.
I made mine with a PVC frame and a sheet of clear, plastic drop cloth. It kept evaporation down and let the light in but, it did not keep much heat it.
I would also suggest trimming the top of the pond with Trex deck boards that you can find at any of the major home improvement stores. They are plastic and are not effected by the moisture.
As a final note, folding the liner inside the pond neatly is not as easy as you may think. I found that placing some water in the pond helped me place the liner by holding it in place.
Good luck!
Colin