We ran a screw into the center of the ring to make sure nothing moved during the next couple of steps. To add a little extra hold, we also shot some brads from the front (wood) side through the plastic and bent them over.
Once we had the basic shape, we finished sanding the pod by hand with 80- and 120-grit sandpaper. If you're going to cover the pod with vinyl, then 80-grit will suffice.
We taped over the top of the pod and used a hole saw to cut the hole for the speaker. We taped it up so we wouldn't mess up the finished body filler. Then we sent the pod to the paint shop. It's important to support the lower side of the ring. The small block shown here was glued in as well.
Duraglass (fiberglass-reinforced body filler) is the best product for this task. Regular body filler will work, but Duraglass, having added strength from the fiberglass strands, gives it extra hold. A lot of emphasis is being made on holding power here. It's important that the bond holds, especially if you decide to paint the pods, where cracks would be a bad thing.
The finished pod snaps into the door just like the original grille and now sounds and looks much better than the factory. Not including the paint time, the whole process took us less than an hour for two pods. The most important aspect of this project is to make sure the angles are the same.
Once the Duraglass has hit the green stage (where the filler is not fully hardened, but not wet either), the pod can be shaped with a file or cheese grater. If you have access to air tools, then wait until it's fully hardened and shape it with a die-grinder and a 36-grit pad. These parts are small, so it doesn't take much.