Shawnisay isn't too bad, as invented names / creative spellings go.
I don't like the "say" at the end, though, it makes me think "Shawni say ___"
Shanisée would have a bit more appeal imo. Why not go all out.
Shawneeleigh, while it seemed cute next to the parents' names, is tougher than shoe leather. But, she could go by
Shawn or
Shawnee.
- mirfak