Another option is to perform a small miracle and part the water (through fix addforce and atom-style variable) 
. After the parting you just need do displace all atoms, so that the vacuum is at the edge of the simulation box. Reset image flags, change boundary conditions to non-periodic and you have your system.
Input file based on the one by @akohlmey
units real
atom_style full
read_data data.spce extra/atom/types 1
pair_style lj/cut/coul/cut 12.0
mass 3 12.011
pair_coeff 1 1 0.15535 3.166
pair_coeff * 2 0.0000 0.0000
pair_coeff 3 3 0.05 3.20723 # carbon-like atom as placeholder
bond_style harmonic
angle_style harmonic
bond_coeff 1 1000.00 1.000
angle_coeff 1 100.0 109.47
special_bonds lj/coul 0.0 0.0 1.0
neighbor 2.0 bin
group water type 1 2
group wall type 3
fix 1 water shake 0.0001 20 0 b 1 a 1
fix 2 water nve
variable d equal 10
variable wall atom (((z-20)>0)-((z-20)<0))*(abs(z-20)<ramp(0,$d/2))*mass*abs(z-20)*100
fix addforce all addforce 0 0 v_wall
thermo 10
dump 1 all atom 50 walls.lammpstrj
timestep 0.1
thermo 50
fix 4 water langevin 300.0 300.0 5.0 645734
fix 5 all deform 100 z delta -$(v_d/2) $(v_d/2)
run 2000
unfix 5
variable wall atom (((z-20)>0)-((z-20)<0))*(abs(z-20)<$d/2)*mass*abs(z-20)/5 #make the wall softer
run 2000
write_data data.walls
