I recently saw this posted as an example of how not to lock things up
Grotesquely unfair, the ring on the inside doorknob is pure genius. It's probably stronger and faster to use than the little slidebar and chain that are commercially available. Plus no drilled holes or other scars on the door. It is easy to imagine a commercial version, modify the ring slightly to cam lock onto the door. Two of them chained together would work instantly on double doors.