Hazel, I have a hard time with the pop-off lids on cans! Your 'coon is talented. You could try one of the "perfect bungees" that LariAnn mentioned (available on Amazon)... from the reviews, they have considerably less stretch as well as being more durable than regular bungees, so they might work to strap down the lid. You could also tie down the lid with a rope, using a taut-line hitch to tighten/loosen, but your 'coon might figure that one out, too.
Ric, LariAnn mentioned using bungees on large pots also. I've also read about using a piece of rebar, driven right down into the pot, through the drainage hole, and down into the ground... but bungees are easier to remove at the end of the season.
