Swimming pool repair water storage bag.
Fullsize image: https://s3-us-west-1.amazonaws.com/h...g_fullsize.jpg
https://s3-us-west-1.amazonaws.com/h..._water_bag.jpg
Previously:
https://www.homemadetools.net/forum/...3683#post99771
https://www.homemadetools.net/forum/...583#post132523