Using a magnet to have quick/easy access in a car can work great where legal.
However, depending on your state/county, leaving a pistol in a car unlocked or unsecured may be a crime.
The low cost and small size of clamshell style car safes, really makes them a great alternative,
to merely stashing them out of sight.

Amazon.com: SnapSafe Portable Lock Box for Guns and Valuables – Includes Lock Box, 2 Keys and Patented Security Cable – an Ideal Car Gun Safe : Tools & Home Improvement
Buy SnapSafe Portable Lock Box for Guns and Valuables – Includes Lock Box, 2 Keys and Patented Security Cable – an Ideal Car Gun Safe: Cabinet Safes - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
www.amazon.com
Yes a determined thief can/will eventually defeat them, but they work great to prevent walk-away theft.