71,
The Polk speaker grills are barely under 7" O.D. just like most contemporary 6.5" coaxials. So if the previous speaker had a larger grill diameter that leaves an indentation around the perimeter then you will face that with just about every coaxial.
Over time heat will relieve much of the cosmetic indentation.
When we've made a dress ring from thin acrylic to conceal the outer mark, we usually pick a color that corresponds with piping in the vinyl. It adds a nice touch.
If for some reason the opening was oversized then a split ring set can be inserted through the opening and pre-attached to the backside. The mounting hardware is concealed by the speaker. This ring set once in place provides the structure to mount a speaker with a smaller hole pattern.
As for '05, I don't recall having an issue. But if you do then the fix is easy.

David
Earmark Marine