Replacing Rear Shelf in 1981 300D - Suggestions?

I am in the process of restoring my 1981 300D. All of the interior will be replaced at some point but for now I want to go ahead and replace the rear shelf, which I have heard is called a "hat rack" also I believe. At some point in this cars history, the rear windows was broken out and the rear shelf got wet and was damaged, along with significant wear on the rear seat. No rust, just seat damage. I was able to get the shelf out in one piece, but it needs to be replaced. What is the best way to do this? I will probably go ahead and modify it to accept some better speakers as well. Thanks!!