There's a police check to exit the subway, another to get in line, a third while standing in line, and metal detectors and ...

Take a look inside the factory where sturdy safety ladders are produced on a large scale! Starting with raw steel sheets, ...

look at how the gate latches shut. Many use a top and bottom latch for extra security, and unless your dog is dexterous, undoing the door shouldn’t be a problem. From there, think about the materials.