Tasmanian Tiger's TacVec Weapon Fixation is an excellent choice for securing a long firearm in a car. You can easily attach it to modular front seat panel (TacVec Modular Front Seat Panel).
TacVec (Tactical Vehicle) is a new line of accessories for transporting weapons and other equipment in a car. You simply connect the holder to the modular panel # 7548 using MOLLE webbing. Thanks to it, you can safely transport long firearms on the back of the driver's or passenger's seat. The holder has fixation points in the place of the stock, center and barrel of the weapon. When ready, the weapon can be unzipped by pulling the red-marked strap and is available in an instant. Thanks to the possibility of height adjustment, the attachment can be used for a wide range of long firearms.