I've heard of putting dry, shelled corn into a too-small leather shoe, then pouring water over the corn. As it absorbs the water, it swells, stretching the leather. I've never seen it done, but it might work, and you wouldn't run the risk of having sand grains stick to the inside of the flask, which looks really nice, by the way.
When the wax on the inside of the flask rehardens, what keeps it from flaking off into your beverage of choice? Or is it a soft wax, not a paraffin-type wax?