Thermostatic boundary layer in hexagonal simulation

I have a hexagonal supercell of an oxide. I want to add a thermostatic region such that the outermost 2-3 atomic layers are in an NVT ensemble, with the inner region in an NVE ensemble.

However, in defining regions I have to define:

xlo, xhi, ylo,yhi,zlo,zhi etc.

However, since my simulation box is not a cube, xlo might be something different on one end of the simulation box than the other. The local xhi and xlo on one particular slab is different than another slab. How then, can I define this boundary layer?


Ahh, prism is what I want.