The rear divider is plywood and easy to replace. I would myself cut into the wood, (or the new plywood) and run a set of 6X9’s and a sub woofer in the trunk. My wife runs a sub in the trunk and it is really nice. That little thin sub will fill in the missing sound really nice. We even turned it down. Powerful little sub.
I don't use rear speakers and if I do I turn down them so I can't hear them, I like the sound in front of me, don't care about the peeps in the back :)