If you have a cheaper pad with fleece or felt on it, you could take it apart remove the filler and insert the pad of choice then just stitch it back together. Or better yet sew on some velcro so you can adjust it as needed and remove the inserts for washing the pad later on. I don't think just putting the pad straight on the horses back will work. You need that space down the spine and over the withers.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France