I believe I have the same issue, and found the builtin plotting functionality of xarray helpful. This worked for me:

p = c.plot.pcolormesh(
    x='x',
    y='y',
    col='layer',
    col_wrap=6,
)

for ax in p.axes.flat:
    projectgebied.plot(ax=ax, facecolor='none', edgecolor='black')