Can you post a pic? How large of gap are you trying to fill?
What I did, which may or may not be the correct way lol, is..
Sealed of all the openings with flashing/ deadener sandwiches. Deadener up to The factory speaker openings, then bolted my spacer to The door. Any small gaps I filled with either expanding foam, or clay.
I then covered the door, and up over the ring with 1/8'' ccf. I then mounted the speaker. I patented had any issues. Mb response is very good, all is nice and solid.
Of course this isn't the only treatment done to The doors, but is what I did related to What your asking. Lol