need advice on shaping sail panel for tweeter

[altec]

I want to install the tweeter in the pic, to the sail panel also in the pic. I am thinking of kind of hard but shapable foam of other material to stick to the panel and shape how I want it. I can do the ring and pull fabric over it but I want the unaffected areas to remain flat. The tweeter is bigger than the panel so it cannot be cut into the plastic. What is your advice and material to use? I tried expanding foam but it remained compressible and could not be shaved. The panel is a replacement BTW. I can fit an alpine srs 610c tweeter in the original tweeter location in the sail panel, but the angle prevents most of what is playing to be heard.

[TK!]

3D printed ABS.

[SR]

You can look at air dried modeling clay