mobile app designer (UI/UX)

I have an existing app (four months old) that is in the early stages. Would like an experienced designer that can help us polish the designs and make it beautiful and easy to navigate! We can do all the dev work, just need help with a simple, clean, elegant design. The job will be ongoing, but will be done in sprints. I would guesstimate 3-10 hours of work a week for the first few weeks, then it could flex up or down depending on the project. Ideal job candidate would be creative, could work with our existing framework, and help us connect with new ideas and customers!