What type of vehicle do you drive? That is going to be the first consideration, IMHO.
Ditto. Start with the configuration of the vehicle and then work from there. Depending upon your resources, there's a variety of locking compartment inserts for today's sporty transports, small to large SUVS and light trucks. Browse the internet for aftermarket accessories.
Whatever you do, mitigate the depth of potential loss and liability by segregating any ammo from the hardware left in the vehicle. For goodness sake, don't give a thief a
loaded gun: if you must leave a weapon unattended in a vehicle, throw your six or eight loose rounds, or a full magazine and solitary .45 ACP cartridge, e.g., in your bag or jacket. Go about your business, return to your vehicle and reload for the drive back to your castle.
Me? I gotta have lots of odds n' ends in my cup holders and nooks - first aid,
, rags (I make my own from old t-shirts to lube and wipe), the usual little boy stuff.
Nuttin' gunnie!
Never!