Crew Rostering and indeed complete Crewing Systems (Crew Planning, Rostering, Control and Crew Records) are by their very nature expensive and are targeted at the medium sized carriers and up. In your case they are not only unjustifiable from a cost viewpoint, but would be like using a steam hammer to crack a small nu!.
I think Fernando Covas' idea of using Excel has considerable merit. It won't be as 'clever' as you would want, but once you have put the effort into setting it up it could prove an extremely useful tool (and would only cost your time).
DIH