Situations with these kinds of sloping walls are unique for each project. There is no generic solution for this, usually the slope is the bottom/underside of the roof you add.

If you want the slope independent of the roof you can use a thin box that you tilt and elevate. A thin box can close a sloping space without sticking out on the outside. Make all sides of the box invisible except the one that you see as the slope.
You can also use a triangle and make all sides except the slope invisible. Rotate and elevate as needed. The effect is the same as with a box.