New shower trim wallplate doesn't cover shower wall hole

I have an old price pfister shower trim that I'm trying to replace. I bought a new price pfister trim so i wouldn't need to replace the valve, but the new trim has a circular wallplate and the old one is octogonal wallplate and when i tried to install the new one i saw that it didnt completely cover the hole in the shower wall. The walls are cultured marble so replacing them is out of the question and i really dont want to buy another wallplate. I'm sure if i could find some sort of stainless ring/washer thats a bit bigger in diameter, i could place it under the new wallplate to cover the hole completely but i dont know how to go about looking for something like this. does anyone know if something like this exists?

Wall plates like that are readily available and any good plumbing supply. Around here, everyone calls them "Smitty Plates". Never knew the reason. The official name is probably "remodel plate". Anyway, it is available.

I looked up remodel plates but I cannot find any round ones. Do you where i could find round remodel plates?
thanks