My recommendations (best first):
- ploopy nano (see my config at Usecase: Thumb Trackball ; I can supply relevant ploopy source code)
- uhk trackball module
- a graphic tablet (I love the feeling, although for mousing it is not extra ergonomic…)
Edit: when writing that thread, I used to have it on the right side, but it works just as good when the ploopy is placed in place of uhk right module. It is about the right height when uhk is supported by the default feet.