Have you considered using a single beefy 10" on 3-4x the power, inverted and the magnet sticking through the seat? You lose a passenger though.

My goal is to keep the system as covert as possible

Check these out.

Pretty sure other companies do the same.

The Excelon shallow mounts that I pm'd you about will do the job adequately for you. You have to work within the limits that you have. One does not have to have copious amounts of power to get good sound, we could get great sound 20 years ago from a 100w x 2 rms amp running a pair of subs. While more power is certainly plentiful now at a relatively good price, one does not have to have all of it to get good sound. The power that you have, 600w rms into 2 ohms is more than ample to get respectable sound in the cab of a pickup.