What is it?

This morning I was walking back from the Diakonia Center after paying for renting their venue for our Christmas Eve service and I walked past Tuol Sleng, the genocide museum. Along and on the outer wall of the museum, they are building this structure, a simple frame with sheet metal over it. I haven’t any idea what it will be when it’s finished. There is nothing on that northern wall of Tuol Sleng, no gates or anything, and I can’t imagine what this little slanted ledge will do.