Fill a gallon jug about 90% full. Then just put the entire jug in the cooler.
I've also seen people use a plastic storage container with a lid. They are more of a problem to set level in the freezer. Again, leave expansion room so the lid stays on.
You can also use 2 gallon ziplocs about half full, and stack them in the freezer laying flat. If you fill them you won't get a flat brick of ice, it will be rounded.
If you put several large containers of water in a freezer at once, be aware that it is probably going to take 24-36 hours for them to freeze solid.