Looks to me like they built a piece that fits the inside of the of the kick. That must be done w/a router to make it fit nice. Then covered it in grill clothe. It's a matter of making positive and negative copies of the jig you cut out to fiberglass into the kick panel area.
I doubt this makes a whole lot of sense, sorry. It's actually pretty easy to do though if you've got a router. I think it looks soooo much better than using the factory speaker grills.