If I am going to carry a hard side container, I would choose a waterproof one. So the first one is out for me personally.

That soft case you link to would be a good choice for a person who doesn't need and desire waterproofness.

The small size is 4¼" x 3" x 1¼"

Quote:
Our indestructible fly boxes are 75% lighter than our adjustable fly boxes. These fly boxes can be stepped on, folded in half, or run over and they pop back into shape. Saltwater fly box features elastic shock cord that keeps the box securely closed.