Hi Jac. I've done this. It works better to use a tiny eye bolt and nut through the lid. Waterproof with a bit of rubber or a drop of silicone. And it preserves the ferro rod.
Maybe I'm not understanding. I read what she wrote as something totally different. She wants to cut off the bottom of the case to put on her necklace. What you propose is keeping the whole case intact and installing an eye bolt (?).