The background is a piece of lightly textured blue Scrapbook paper. This is glued to a piece of gray chipboard (like the back from a tablet) with 3M 77 and then that is glued to the back of the workpiece with the same adhesive. I then use glazier's points to make everything fit nice and tight in the frame.
Very wide selection of papers available at your friendly Michael's, Hobby Lobby or JoAnn's stores.
You can get chipboard at any print shop/copy shop.
~~~GB~~~