I used a curved sanding block and flexible sanding board to get a nice even "arch" on the wheel well opening. Then, I used some 60grit or 80 grit sandpaper to put a nice radius on the lip. I think I started out w/ a small rubber block, then as they got rounder, I just used paper folded up in my hand. There were a few areas where my fiberglass was about 1/16" thick at the lip!
I just layed up a few layers of fiberglass mat to build up the lip and radiused as described above.
In the louver openings, I used a file to take the cutout size back to the molded edge, then used sandpaper to radius it.