In a pinch, use a brief case. It's not great, but better than nothing. I prefer to never keep a gun in the car.
Can think of few things more likely to be stolen in a smash and grab, or any vehicle burglary, than a brief case.
DO NOT leave gun in briefcase. Don't recommend you leave the brief case, for that matter.
I use a Lock Box secured with a wire. It hides under the seat and is hard to see, since the wire can be covered up by the floor mat. If it is spotted during a smash and grab, it would take bolt cutters to steal it.