that’s a really smart idea! As someone who’s very small busted myself, I totally get this. I’m a little unsure, myself, on the correct way to accomplish this while still maintaining the shape of the sweetheart neckline, meaning I would most likely try only adding the elastic to the back of the dress or the side of the bustline, rather than the front of the dress. It just seems easier but I don’t know if it would have the same effect. In order to create an elastic tunnel, you could use ribbon that’s sewn to the lining, near the edge of the top of the dress. Then insert your elastic and stitch one end of the ribbon closed with the elastic also in the ribbon seam. Then stretch the elastic as need to the other end of the ribbon and again stitch it to the end of the ribbon. I hope this helps a little bit, but if you figure out the perfect recipe, please let me know. 🙂