I don't know if it would work with bangles, but a friend has a ring that was a bit big, and instead of making it smaller they put some "bumps" on the inside. Is there anyway a jeweller could do this for you, or add a layer of something to the inner part of the bangle? Maybe a layer of lacquer if heating with silver is not an option? So in effect, making the circumference of the inside less, and the bangle thicker...