You need a grid with less than 1/10 wavelength holes, or about 2cm in case of Outernet.
The grid has to have conductivity in all directions. So a closed sheet is of course OK, but
when it has small round holes (like the one shown in another thread) it is fine as well.
The grid shown above is OK in one direction, but in the perpendicular direction the spacing
is much more than 2cm so it does not reflect.