Here is a step by step of how I am mounting my grilles. This will allow me to remove them for paint if I ever get that far.
First step is to cut the perforated material a little smaller than the trim ring.
Next lay both pieces in the recess and line them up so that they fit well and no perforated material is shown around the edges. Mark any 2 holes through the perforated material.
Remove the trim ring and drill the holes you plan to use to bolt it down. I drilled them the same size as the existing holes. 3/16 diameter.
Put the trim ring back in and mark where the body needs to be cut.
Next counter sink the holes that you transferred to the body. I used 6-32 flat head screws.
After all the holes were countersunk, I powder coated both pieces semi gloss black