Small bag

Take the tetrabrik and open only the top 2 corners. Cut the top side without separating it from the rest. You can now rinse the brick and let it dry. Then cut into each fold of each corner a vertical notch to the foldable upper edge. Then fold the lateral and front sides of these notches inward.
 
Now all you have left is the back side, which you can easily flip to the front. Glue a piece of Velcro on the inside and make sure that it matches with the one you stuck on the other side. Then fold the lateral sides of the tetrabrik inward so that they are well aligned. Finally pierce two last holes on the upper side in which you will pass a ribbon that will serve as a handle.