Use a piece of 1/4" plywood for the back and some 1x1 or 1x2 boards to make a square frame and then add your plywood to the back. You can make or mount a frame on the front if your using it in the house. I used to paint the inside of the box black to show off the contents better.
Like making a wooden box with no top, just attach a frame. Figure out size you need and watch the you tube videos on making shadow boxes. Have fun.
Hi Susan, hope this video helps you