I've always wanted to make a sandwiched lunch bag - meaning cotton+polar fleece+cotton - to make it water proof. My lunch box is not air tight so there's always a slight chance for liquid to escape and wet the lunch bags, which were made of 100% cotton.
Fabrics cut out, the polar fleece from samples I freecycled from a kind lady in Bexley, and the small flowery fabric bought from a charity market:
My planning (I'm too lazy to type it out...):
Final product (of course I can use the other side out too.):