I am a little confused as to what it is you want to do. You want the grills that came with the speakers to be flush with the body panels in the car? The only way I can see doing that is to cut a hole in the panel and then figure out at which depth the speaker/grill need to be mounted in order for it to sit flush. I don't think I have every seen anyone try this but I'm sure it can be done but I don't know how it would look. Using this method the hole would need to be cut to the exact size of the speaker at the same angle it will sit when mounted. Good luck with this, post some pics when you are done so we can see it.

97 RS- Runs, but never driven
2000 Subaru Outback- Daily Driver