With the TPMouse script, I implemented the activation shortcut as [LShift][RShift][C (trackball mode) / G (grid mode) / Q (quit)], which I felt had a nice balance between deliberateness and easy-to-reach (since you are using it with your hands on homerow).

Though because some keyboards have key rollover issues with using both Shifts, [Capslock][<modekey>] is also allowed as an alternative activation shortcut.

[0] https://github.com/EsportToys/TPMouse