As I mentioned above the dowel sticks and blackout material have worked well for me for over 35 years. You could also consider the black plastic garden sheeting. I used that in a similar way to black out the two windows in my darkroom. It is pretty heavy and you could use a double layer. The nice thing is it can be placed or removed in a minute and when rolled up on the dowels like a scroll takes up little storage space. Make it long and wide enough to more than cover the door. The sides can be secured with self-stick Velcro.