Currently there is only one form factor: set next to the keyboard held into place with a heavy base.
How about something similar to the iCade mobile, except with a 3dmouse instead of a thumbpad?
It shouldn't be heavy, as the only reason the Space Navigator is heavy is because of the heavy metal base used to prevent it from moving.
The device itself should be cheap to manufacture, as it would basically be just a harness not constructed with any electronics.
The 3dmouse and harness would have to be designed to be locked together, or not be modular and simply constructed as one piece.
I've described it in more detail here in a separate page (as the page can be revised from time to time.)
http://sites.google.com/site/forceinput/form-factors
The power of a 3dmouse coupled with the power of a smartphone, all portable! (No need to sit down at a desk to use.)
Link to the iCade mobile: http://www.ionaudio.com/products/details/icademobile
