Start by preparing the bottle. Remove the labels and rinse it out with water. Cut the bottle along the seam. The bottle will be thicker at top and bottom so you’ll need a pair of heavy shears to cut it apart. However, you could easily cut out the center with a normal pair of scissors.
Use a permanent marker and ruler to lay out a rectangle. Cut it out with scissors. Next, round off the corners. I used the lid of a cinnamon container to mark the corners before I cut them.
Roll the plastic up, long ways, with the sides towards the middle. Use rubber bands to hold in the rolled shape. Drop the roll into a pot of boiling water and leave it in for about 30 seconds. Use tongs to remove the roll before running it under cold water.
Remove the rubber bands. Place the bad buddy under the handles of a plastic bag to use it.
Being carried in the picture is two full gallons of water, double bagged.
When it’s used, the very edges will conform to the bag handles. It won’t cut through the bags and the shape provides plenty of strength to keep it from folding in half.