Its easier than you would think to get the horse to want to "play". I think my horses main problem has been I use a curb bit. I am going to try a snaffle next time we play and see if she will push it. The shanks on the curb hit the ball first and it puts her off. I have seen police horse training where they use a big ball to teach the horse to push against crouds.
You really should try it, its a lot of fun. I think any large ball would work.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France