@
Dadhawk
I think you could fit a 'permanent' removable wall with the seats still in place, as behind the seats is just styrofoam and the trunk mat. With a bit of trimming, could have a piece coming out from 'under' the seat, and mounts to the same U bolt that the seats connect to. The issue there would be how to remove it easily when you need the seats down for cargo purposes.
You might be able to 'slot' it on the bottom, and use velcro as @
Ultramaroon suggested on top. That way it would support something hitting it (a la Luna) and still easily removable. Pretty sure there's a lip on the styrofoam to support that. I don't recall offhand if the trunk mat and styrofoam end at the same point however. Might still require trimming of the trunk mat just get the divider to fit. I haven't had back seats installed in some time, so I don't recall how much space there is between the seat and the trunk area.
I have a bunch of photos sans back seat, but I don't seem to have one in that particular area. I will be taking out my rear seat delete to modify a couple things on it, so I'll try to snap a couple photos then so you can see how that area looks.