Relatively thick (13.5 mu m) Bi substituted ferrite garnet single-crystal films exhibiting wary magnetic domain structure are prepared. It is found that domain wall waviness affects the magneto-optic diffraction (MOD) pattern resulting in the appearance of the additional diffraction maxima characteristic of two-dimensional diffraction gratings. The observed MOD parameters are adequately described by the Raman-Nath thin layer approximation to the theory of diffraction.