My girlfriend's fingers are becoming slightly thinner with time. Her ring is now a bit too big for her finger. She tried to switch from one finger to another. The two thumbs are too big (and it would look kinda ugly on the thumb) and the 8 other fingers are too small. The ring cannot be easily resized down further more due to its design and as it is not a very fancy ring, we don't feel like paying too much (or even anything!) to make the necessary modifications. Its value is sentimental.

How can we make sure the ring doesn't get lost?

We thought about an elastic to make sure it doesn't move but it is ugly. We thought about adding stuff in the interior of the ring to resize the internal part of the ring but couldn't find something that worked fine. Do you have a trick?