Walls below stairs

[Bill B]

In this model, there are two sets of stairs. One (consisting of two flights separated by a landing) leading from the basement to the 1st floor. Two: A second set identical to the first leading from the 1st floor to the 2nd floor. The 1st to 2nd floor stairwell is directly above the stairwell to the basement.
Can someone point me in the right direction to create walls that are BENEATH the stairs leading from the 1st to 2nd floor and follow the stairs? I tried increasing the stringer bottom but that simply fills in the small space on the first floor but drastically extends the bottom of the flight to the 2nd floor so that is pokes into the hallway.
The final outcome is one in which the walls follow the stairs, then an interior wall and door leading to the basement stairs. Please see the attached JPG that should give you an idea of the challenge. Thanks, Bill B