I found a broken bunk bed in the trash. I used the side rails to make a box to block the entrance to the hall.
I re-used the screws from the bed.
I put some 1x4 inch plywood on the back of the top and some mirror squares over that.
I used a piano hinge to attach it to the door jam.
I finished off was a handle on the inside and glued some magnets to hold it shut.