Here are a couple of suggestions for filling holes or seams in scenery. Dutchman. You can pretty much get away with white glue and strips of muslin. Just thoroughly soak the muslin in a white glue /water mixture 75% glue to 25%water and smooth the strip over the seam. Joint Compound or spackle. Works great on smaller holes and tight seams. Don't go too thick on your first coat it will take forever to dry. Bondo. Make sure you have a well ventilated work area for this stuff. But it is amazing for filling in the missing pieces.