rear firewall

If you wanna keep speakers and a backseat then put the firewall in the trunk and make it semi-premanent. What I mean is, drop steelangle, or flat stock, down from the trunk lid seal area and attach a sheet of your favorite metal from the wheel tubs and inner rear quarters, to the trunk seal and floor, with aluminum rivets, or Dzus fateners. Might even consider making it 2 pieces with a lap joint in the center, under the rear window. Easily removable and yet seperated from the passenger area. If it's done nice, the tech guys'll be so impressed, they'll leave the rest of the car alone.