Dirki:

You can't "add" things to floors or ceilings.

You could use a texture that you add to the ceiling?
You could google for a 3D model of a rhombus.
You could make the rhombus yourself using a 3D program like Blender, Art of Illusion, or Misfit 3D.

In each case you have to give the rhombus the required elevation to make it "float" underneath the ceiling.

I hope this answers your question.

Hans