To prevent any telltale rings where the sat nav had been mounted, I formed an Aluminium sheet to the shape of the dashboard, and clamped the sat nav to it, along with a lump of steel to add some ballast.
To the underside of the Aluminium sheet was glued some "nonslip" rubber sheet. It takes quite a big bump to shift it even slightly.
After use, it can be locked in the boot, (trunk) or taken with me, leaving no trace that there might be a sat nav somewhere in the car waiting to be stolen.
Howard