Summarize:

You could cut it curved or straight, however you prefer. Glue or staple gun handles on each side of the top of the water bottle. Suitable handle ideas include:  Leather or fabric straps Plaited twine or sisal Rubber lengths Braided fabric Old belts Anything else you can find around the house. Make the base of a bag (back, front, sides but not top); reserve the hot water bottle as the top of the bag. The base of the bag should be wide enough for the bottle to reach over from the bag back to the front, as a “lid”. Place a bag clasp into the portion of the bottle that was once the top. It’s quirky and will require some daring experimentation but it can look very different when done! In some versions of this bag, old military truck tarp was used as the back and sides of the bag.
Cut the top of the bottle off. Attach handles. Another way to turn the hot water bottle into a bag: