There was a cardboard panel installed behind the rear seat in the factory, probably to hide the springs and innards of the seat back from view in the trunk. The only reason I can think of for the panel not being solid is to keep the weight down without compromising structural integrity. These cars make great bracket racers because they are so light.
When we did my car I put a replacement divider in to make it look like it did when new from the back.
Inside the car I put Hush Mat from the upper firewall area all the way to the rear glass. I put it over the cardboard divider on the inside. It doesn't matter how hot it gets outside I car run my AC very low and still cool the car.