Hi Al,
Give this a try. My assumption is that the landing is ‘offset’ , so you do not have a full height wall that is wide enough under the landing to insert a door. The door is falling at the break of the angled wall and the full height landing wall.
The angled under stair wall must be broken and angled. There are no changes in this method to those steps. Break the wall, set slope as needed. Leave full height section full height.
Instead of inserting an opening, insert a NICHE. The niche can be set to a shallow depth of maybe 2″. It is sized to be the size of a door, say 36 x 6’8″. That should span the break.
Now take a member 2×10 ? , rotate it to create the angled wall appearance. Select it , right click , select adjust to plane, click on option to set to Degree and something around 41 deg to 43 deg should come close to stair angle. Now elevate it . Assign wall material.
The NICHE is just a recess in a wall so it has ‘drywall on it’s face. Use that face with the paintbrush to add a wood grain.
In Interior Accesories under Architectural Details are some Door Knobs you can add to make the door look real.
In plan view you would have to mess with the 2D lines / adding a swing for document purposes.
Break / Add Niche / Add Slope by Degree Member / Paintbrush and Door Knob.