You have the basics of doing it in a shadowbox, but we need more information. Do you want to use a battery or corded light source or a non powered light.
For a non powered light, you could mount the glass photo negative on a pedestal frame(cradle) with 4 legs and a hole the shape of the image in it to support the negative over a mirror or alum foil that will reflect the ambient light onto the back of the image.