On the rare occasions when it is necessary, I use one of those small "gun safe" boxes cabled under the seat.
While some people strut about proclaiming how useless they are, it satisfies the requirement for "due diligence" and protects it from casual smash-and-grab thefts. If a thief has a battery-powered angle grinder and a prybar, it doesn't much matter how you store the gun.
Whatever method you use, keep in mind that thieves know about GFZs too, and if they see you twisting about inside the car, walking to the trunk, etc., it's pretty obvious you're leaving a gun behind when you walk away.