G'day,
I'm an experienced freelance Ruby on Rails programmer.
For the last five years I've developed relationships with a couple of long term clients doing 5 hours here or 10 hours there. It sounds like that's something like what you're looking for.
You specifically mentioned that Heroku experience is required, which is fine with me; I've worked on several Heroku hosted RoR projects. (Only very recently did I help my most consistent client migrate from Heroku to Azure)
You haven't described your app in depth, but given the variety of RoR apps I have worked on in the past I've at least briefly touched on most of the major technologies, libraries, and gems that might be associated with a RoR app.
I typically prefer to bill hourly, but I'm willing to estimate my hours in advance, and provide a fixed price per project.
My hourly rate is somewhat higher than is typical for Freelancer.com, but I produce good quality code consistently, and tend to get more done per-hour, due to my experience and skills.
I'd love to hear more about the specific project you want to work on.
Tim