How to make molecules fixed in a plane?

Hello, everyone!

I work on a simulation of adsorption monolayer of trimesic acid molecules on a flat solid surface. It requires too much computational time to simulate the surface explicitly. Actually, I only need to constrain the molecules in two dimensions. How to fix the molecules in a plane or some thin box without boundary conditions in z-axis. I plan to do that with a simple quartic potential a(z - z0)^4. Is it possible to do with MS/Forcite? If yes, how?

Thank you in advance!