I would use .020 or .015, it really depends on how stiff you want the flap. If for a soft bag you may want the flap to bend in certain places with a natural looking fold. In that situation you may want a lighter .015 Bontex. You can also use Pellon to make the stay too. giving it a really soft but it still retains the functionality of a stay. Lesson 4 shows the flap being made and it’s in the “shorts” section too.