Take a look at the last step so you can visualize what you are making before you begin.
Step 1: Map the curve of the TV
I took a piece of scrap paper and cut it to the general shape of the curve, making sure to leave extra paper on all sides so I could trim it down. I then put it behind the top of the TV where the speaker holder will sit. I then trimmed the paper until if fit the curve. It will be hard to get it perfect, as long as it's close it will work.
If you look at my paper below, the top is where the speaker will sit, the right side is my wall, and the left side is the top-rear curve of my TV.