thanks!One of these is 0.36 m2, so 1.4m2 for 4. They look nice and membrane absorber is the way to go, but you might calculate the size of your back wall (or front) and then compare. It will increase (average) wall absorption by about 0.2 if your wall is 3 x 2.5 m.
So be prepared that you might want to use more of these. But your pictures show that you are already planning that way ;-)