I have one of those 'one gun safes' with electronic lock bolted to the floor of my truck. I bought mine specifically for leaving a gun in the unattended vehicle. It's pretty secure [it could be removed with a big hammer or pry bar] and easily accessed by pushing a few buttons.
To stay legal, if you do not have a WA CPL, get one. Without it you cannot legaly leave a gun in your vehicle in Washington State. Get a Utah permit too. Then Oregon, Colorado and California are about the only places in the West you cannot carry. And for the gods sake, do not store ammo in the box with the gun if going into California!