I generally try to avoid leaving a firearm in an unattended vehicle. However, there are occasions when I need to lock up a handgun while making a quick trip into the court house, post office, airport, etc.
For these times when the firearm will be locked in the car for a short period, I use a simple "secure-it" gun safe with a cable. It isn't the strongest lock-box in the world, but it will make a thief have to work a little harder than just "smash-n-grab." Since I do not leave a sidearm in a vehicle for long periods, this works for me.
I also will use it as a lightweight travel safe for securing passports, cash, credit cards, etc. that I may not want to carry with me when stepping out, but want to keep under lock & key back at the hotel room. Again, not the most secure option, but offers some increased security for items left in a hotel room.
Large Pistol Security Box 9-1/2 x 6-1/2 x 1-3/4 Steel Black