I have started keeping a tote bag in my car with several sizes of containers. When I go to a restaurant, I take my tote with me and then I can take my leftovers home in my reusable containers. If you use stackable containers, the tote won’t take up much room in the trunk.
It also helps to have someone in the family who is willing to go out to the car and get the containers when you forget to bring them in!