You should know that Trotec tells a lot of metallic engravable materials, including gold that can be engraved to reveal black. It would be another way to do a project like this. It’s probably easier than using laser foil, too.
For example:
I think they have some stuff that is even thinner, like a sticker.
Unfortunately it can be hard to get their stuff a sheet at a time, they are not geared to support hobbyists.
Gravotech has some similar stuff and I have ordered a sheet or two from them in the past.
Good luck with the project… hard to imagine a 78 year old MIL who digs Hellraiser but I love it!