double adult busmar - Panther

i've got dads panther now, im thinking of getting a double adult busmar to fit onto it - make it into a little caravan type thing, how cool to not have to pitch a tent etc, just open to door, drop the seat and lay down in the dry [ok not lay right down but i can sleep in a chair so reclined would be fine]

anyone know where there's one knocking about cheap?