Do what the FBI does down in DFW. They secure some heavy duty chain in the spare tire area of their crown vics, and then have a loop or some other fastening device that they thread the chain threw in the trunk lid. So to unlock their "secure" trunk, they have to reach into the truck and unlock the padlock, that limits the opening of the trunk to a 4-6 inch opening... just my .02
Handcuff it to the seat frame
I have a relatively inexpensive .38 that I keep stowed in the vehicle. If it was stolen I don't have to give up eating to replace it.