I'm have put a plane onto the same height as the edges of the cube are. Everything you see was created in Blender and you can download Blender file here. The plane is a little bigger than the hole so that they overlap.
The whole rendering is a little funny. I get this frame around the hole due to plane and cube edge having the same hight. I only want the plane to be visible. How can I fix this?
EDIT: I can always change height for a tinywiny bit but I would prefer a different approach due to shadows and reflections and stuff.
I'm a little confused because you are referring to a hole while it seems that your cube does not have any hole and your are adding a plane on top of it.
What you are seeing is called depth fighting and it's because both objects have the same z-value, yes.
SCNMaterial exposes properties like
readsFromDepthBuffer that can help with that. Also check