There's always ways to McGyver installation securely if you need to.
I have a Nano Safe in my Jeep. It comes in handy on occassion for other things also. I installed a Bulldog safe in the wife's Jeep, only because they were out of Nanos in my area, and it works just as well. The main difference is that the Bulldog has a separate mounting bracket along with the optional cable.
As I've stated before from experience on the streets, if someone wants something badly enough, they'll find a way to get it. Especially if they're a pro. The trick is to make it as hard as humanly possibe for them. The $25 investment in the safe is cheap insurance. Another way to minimize the risk is not to be too obvious about what you are stashing wherever you are doing it. Consider stashing the gun the same as concealed carry. Do it covertly, or at least as much as possible.