Shower Controller replacement problem

[Golfish]

I am replacing the existing single handle shower controllers in my house. There is no rear access to the controllers so I have to do everything through the fiberglass stall hole cut by the installers in 1979. I need to cut the existing hole larger ( about 12" dia.). Does anyone know where I can get a larger cover plate or another way to do this?

[Reedwalker]

You need a renovation plate