On the back of the shadow box, measure around 2 inches down from the top and 1½ inches in on both sides, marking two dots. Then, squeeze a generous amount of glass glue in these areas and secure ...
All it takes is a SANNAHED shadow box frame, which only costs $9.99 ... If you're worried about the shelves not being secure, you can glue them in place with a construction adhesive like Liquid ...