I finally found a way to re-use those big 50lb bags that dog food comes in. They are plastic coated paper, so they shouldn't be burned, and yet they're so big they take up tons of room... plus that coating and all that material just seems like they should be worth something.
If you're like us and put your dog food into a different container than the bag (we use an old fermentation crock from Redwing, MN)
You have these empty bags. I figured out that if you cut them open along the side - the long side - after cutting open the top in the typical way to pour it out, I inverted the bag so the shiny part was inside and stapled up the short end with a typical office stapler. Then I filled it with dirt and added a squash plant!
It's working really well. it's not water tight, but that's not what you want anyway. I inverted it in part because I didn't want the label showing. We do all container gardening to save the water and keep things away from the gophers, so this is perfect.