In ase, if I want to create an oriented structure, with certain miller directions along xyz axes, I do the following:
from ase.lattice.cubic import BodyCenteredCubic
atoms = BodyCenteredCubic(directions=[[1,0,0], [0,1,0], [1,1,1]], size=(2,2,3), symbol=‘Cu’, pbc=(1,1,0), latticeconstant=4.0)
This sets up a slab of BCC copper with  along the first axis,  along the second axis, and  along the third axis.
How do I do this in pymatgen ?