Gluing resonant objects inside soundbox?

Hey folks I was hoping someone might have some insight on a thought I had. I've been thinking about trying to glue things like cookie tins to the inside of the box of a dulcimer, either gluing them to the underside of the soundboard, or gluing them to the inside face of the bottom board. I know that thin metal tins like those tend to be very loud, but with a harsh tone, so what I'm hoping is that by putting it inside the box, it will add volume, and the wood box around it will absorb most of the harshness.
I'm still having a hard time grasping the physics. If I add lightweight metal sheets, or anything else, into the inside of the box, will that just prevent the air from "agitating" and make the whole thing quieter? Does anyone have any insight about putting things inside the soundbox, and if that affects tone or limits volume? It's kind of intriguing to think that you could shape the tone of a dulcimer by adding a specific set of objects to the inside. Hoping to get some experiments going soon
Thanks
Nate