Honestly, excavating a hole in the ground and then burying them alive would be the peak of efficiency. Make a channel so when they walk in they are not immediately alarmed, then seal the exit and push it together. A couple of improvised machinegun positions every 100 yards should deter climbing. EDIT: I'm thinking really big guys, they could use the propaganda of it being a fallout shelter. Or vaccination site from the next scary fabrication on the TV.