A search should find more info. I know I've responded on this same question.   There is likely a better/easier way too.   Draw a normal wall. In elevation, drag it up and shape to conform to the stairs.   You will see the framing visible on the bottom of the wall.    Shape a solid, sloped soffit or ceiling plane as I have to cover.   I changed the wall to a single drywall layer which may be good enough. (needed to use a ceiling plane)  
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