Got some time to work on it again. To float the rings, cut some small pieces of MDF. You need to scale the edges so that it fits the edges of the box properly and keeps things structuraly sound. To hold all the pieces together, you can just use high temp hot melt glue. The nice thing about this is that once your done, you can heat up the posts and pull them out later. Make sure things are strong because the fiberglass resin will put a large amount of force on the whole thing as it cures. I have seen a few peoples boxes colapse as it goes.
As you can see, i went for a 3-D shap that tilts them slightly forward and up.
Seems to be a good fit in the car.
So now, we must streatch fleece across the top. To hold it on, for the most part you can use spray adheesive. I coated the upper edges all around and covered the ring. However with this design, it wasn't sticking to all the edges, so i used a staple gun to make sure it would stay. You must streatch the fleece very tight and then trim off the rest.
Then your setup should look like this. Don't mind the indentations, we'll fix that up with body filler later.
Just like before, we use fiberglass resen, only this time we soak the fleece. No point in doing the excess on the sides or the middle because all that will be done away with anyways. Just coat things nice and well and make sure you get it into all the little spots. The more you use the better and let it harden like before.