Thanks for posting this. We just reinforced the box/building where we keep our feed & scratch, with some thin sheet metal. SQUIRRELS have torn it up! They chewed through the door (after we blocked a vent hole they enlarged, with a prior piece of sheet metal) and chewed holes in the covers of the plastic buckets where we actually store the food. Two mornings in a row, a squirrel jumped 
out of the scratch bucket!
I hope that's the end of it, but I've been pricing metal pails and trash cans online, with the idea that I'll need to find another storage solution later.
I was thinking of attaching two bungee cords to both side handles, twisting them around the handle of the cover. Of course, bungee cords are chew-able, so I actually thought about those wire-like bicycle lock cables. Then, I see you are planning to use chain (or a cable similar to my idea) and you already have the eye-hooks (if that's the right term) I also thought about. It's almost surreal to see "my" ideas already thought out and implemented! 
