You could always buy a large piece of fleece and tie it or just get a piece and finish the edge using some kind of fleece that reflects his personality!!!
Not sure if there is a available but how about a cd case for his car? If he is having a large party just make a small wallhanging or pillow case/cover using fabrics that show his interests or school colors leaving plenty of room for all of his "friends and families" signatures and wishes for his birthday!! You could applique Happy 18th and use birthday fabric.