That is an interesting discussion. I'd go for a small door (opaque)/large window (transparent) that one can from the inside. A large triple glazed window ON the door (a smaller door ON a bigger door), a red/yellow light switch, a elevator-like ring/bell, a walkie-talkie permanently installed inside, a staff-count every 30mins. And then I am thinking that after 2-3 weeks all the people-dependent solutions would be abandoned...