Axelmars:

If you need TWO ceilings, you can simply make TWO rooms inside that one large space: one with ceiling ON, one with ceiling OFF.

Second way is, indeed, to insert a flat box. Advantage of this option is that a box is opaque from both sides; ceilings are opaque only when seen from below.

With oblique you mean that the ceiling is not a perfect square? Use one or more triangles around a box to form any other four-sided shape. Rotate the triangle while importing to get the correct orientation.

Hans