Yes, a lot of the first line defence bunkers where i live had a brick outer wall built around then and real roofs. The wooden panels covering what should have been a window would just swing open just to reveal a mg or at gun. REALLY good camouflage you could say. At this location we also held the German army for a whole 3 days, just to hear the country had surrendered.
Those bunkers where build prewar, and were finished at around 1935, so i guess time wasnt really a problem then.