I would plait her hair with rubber bands then I would fold the long plaits up into normal 'bobble' plaits and secure them with insulation tape, white tape for white horses and black tape for bays etc. Or you could go random and use green tape? Or you could alternate black and white or you could use tape in your cross country colours (at a one day event). It looks really classy, is a lot quicker than normal plaiting and is super easy to take out afterwards! I do this for small shows or formal riding lessons. For a big show, I would definitely sew my plaits with thread though.