???

 

The quick and easy solution:

Lucid Automotive

These folks are a Mom and Pop shop who crank out stuff to improve audio systems for the s2k. If you do some searches in this forum there are far more sophisticated examples of how to add rear speakers to your car, but it's all a question of what level of results you seek and how much effort you are willing to expend to get there.....

 

FYI, those panels are pretty easy to cut yourself. Took me 5 minutes.