When i changed my speakers i needed a mounting bracket for new speakers. Wood brackets are not the best option. I found plastic ones but again universal (round only) and sometimes they wont fit in some cars and these days finding good 6x9 speakers is not possible in good brands like hertz etc
I found a video of a guy using acrylic to make a custom bracket according to the car and fitting a 6.5 into it
I guess number plate shop would be good. They have router bits too to cut those edges smooth so i guess they can cut a custom shape too if i give them a template made from cheap wood