I just rest it on the dash behind and in the gap between the instrument console and to the right of the air vents, it remains in position with out any fixing and can be lifted off when not in use.
We mostly sit ours in the area above the central air-vents. Sometimes closer to the passenger, sometimes closer to the driver. In fact, it'll sit almost anywhere on the dash without sliding about. It's a Garmin beanbag and has a rubberised, non-slip underside. (I don't know how the TomTom version compares.)
Just love its convenient portability for a number of reasons. So easy...