Then Lisa remembered from her year long stay in Ghana the so called "rubber bags". Such bags were handed out by vendors at the market. Soup was also sold in it, showing such bags could hold water without leaking. Such bags are also used the Netherlands for the same purpose.
Result: I asked the girls to sew a cloth bag same size as the plastic bag as to give it support and protect the plastic against punctures. At the bottom of the cloth bag I fitted a wooden board with the self-closing valve.