Sit behind another player and use a safety pin or paper clip to afix the music to their shirt. The old sopranos have a mount for the lyre and you can use the mast part of a clarinet lyre to give the proper angle. In a pinch, you can have a mount epoxied on the horn but as mentioned, it will leave a mark! I would consider taking a clarinet lyre, unscrewing the round part, drilling a hole in a clothes pin or some kind of clamp and mounting the lyre to that. Then you can just clamp it on the bell. I am assuming it is a straight soprano. If curved, more improvising!