Simulation Box with Two Different Widths

Dear Lammps users,

I am trying to regenerate the results of a paper which investigates the heat and mass transfer characteristics of a nanoscale evaporating meniscus. I have attached the figure of the computational domain and as you can see the liquid argon receives heat from the bottom platinum wall (red wall) and evaporation occurs so a concave meniscus is formed by placing liquid argon between a lower wall and an upper wall, with an opening in the upper wall.After evaporation occurs, the argon particles immigrate to the vapor region with a width as equal as the width of the opening.

My question is that how I can confine the argon particles from immigration to top of the side top walls (blue walls.

I tried to define two different regions (box1 & box2) and combine them together (region box union box1 box2) in order to have a simulation box with two different widths but it didn’t work and argon particles filled to top of the corner top walls.

Thank you for your help!