I have a similar problem. I drive a club cab pickup truck, so I don't have a trunk. What I did was I bought a First Alert Electronic Digital Security Lockbox model 3040DF. I found it at my local national hardware chain (the ornge one, not the blue one) on sale for $40. I bolted it to the floor in the back seat. It would be a bear to take the whole thing out.

For years, I've been plagued with the overwhelming desire to run motorcycles off of the road a la "Spy Hunter."