I usually use white foam core behind the plexiglass. But...if the design on the glass is dark, and there are scratches, I use black foam core instead.
There are a few scratches in the dark part of this glass, but the black foam core made the white part of the design look shadowy, so I used both colors. 😊
I didn’t put the wire on them yet...Hobby Lobby forgot to put it in the bag 🙄