I tried that, worked for a while then they figured out how to push the rocks/bricks off. I then tried a short rope which required unhooking to open the bin...

I tried that, worked for a while then they figured out how to push the rocks/bricks off. I then tried a short rope which required unhooking to open the bin... they chewed through it immediately. The only thing that worked in the end was a metal chain secured to a bolt.