Does anyone know how to create that beveled edge shown on the coffin top?

I'm wondering if that lid isn't styrofoam or layered foamcore covered in fondant. The styrofoam would need carving, might not be as exact as this. I'm leaning toward progressively smaller cut outs of foamcore board on layered and secured on top of each other and then it's covered with the fondant.

Or 3 gumpaste plaques placed on one another?

I'm going with both Khoudek. 3 pieces of styrofoam, stacked on top of each other.