Not sure where the gas tank is on a dodge charger, but with most cars it's under the rear seat, taking up most if not all the usable space, even if you cut through the metal. You can google hidden safes in cars and get some ideas. The best "universal" one I have thought of over the years is a very shallow sheet metal box mounted on the ceiling of the trunk. Paint it black and have it hinge down, or a drawer style that slides out. Not easy access but custom built boxes like that should be very tough, and secure. I haven't ever had one quoted, but I imagine it wouldn't be too much more than whatever you could expect a commercial version to cost. A sheet metal box that size would be a small job for an industrial sheet metal company or something similar.