I would try to sculpt something. As they are large models they need Large Bows. Start with an armature (i use paper clips generally) for the general shape and then secure the middle of the bow with green stuff and wait for it to cure completely. then in thin layer build up to the design you'd be happy with. Because they are Nurgly you can get away with imperfections. I wouldn't bother with Bowstrings as they will likely just break.