The two best ways to finish a fiberglass box would be either vinyl or carpet. However, the best way would be carpet as it is much more forgiving than vinyl. The fiberglass box does not have to be perfectly smooth to apply a carpet finish whereas it does for a vinyl finish. The carpet finish is more resilient to scratches than vinyl. However, vinyl looks better in a modern surround than a carpet finish and it is much easier to clean if a sticky liquid spills on the fiberglass box. Another alternative would be paint; however the surface needs to be perfectly smooth and a large amount of prep work is required so that the paint adheres to the surface.