Here is my ole blue HB when I started aero modding....
First the lawn edging spoiler, I was one of the first to do this around 2001
After having issues not keeping enough temp. in the engine in the cooler months, I used roof flashing painted flat black to cover the opening, then had to trim as it needed some air. (nevermind the bumper, basketball in sub zero temps = cracked!)
That is when I got the shutter idea.