PBC along xy direction Only at z=60 and z=-60

GROMACS version: 2020.1-Ubuntu-2020.1-1
GROMACS modification: Yes/No
I want to apply PBC along XY direction only. For this, I need to create sulfur walls at z=60 and z=-60. To apply the PBC along XY direction, we have an option pbc= xy. If we set nwallls=2, it creates walls at z=0 and z=z-box, but I need to create Sulfur walls at z=60 and z=-60. Please suggest how to create walls at desired points?