I would source some regular speaker grill cloth, cut to fit, and secure with some contact cement at the very edges. Not sure if you can get this locally anymore 'cause Radio Shack doesn't carry hardly anything anymore. Check online for a speaker building supply website. It just needs to be something "acoustically transparent" so as not to block the sound.
As for paint, I don't know what the factory used, but when I do touch up stuff on trim pieces, I use the Wurth Satin Black Trim paint. Excellent results so far and in some cases, probably better than original.
Funny...this exact project is on my list of things to do when I replace the speakers in the Coupe!
- Darryl