In the Ultimate Car Audio sticky at the top of this forum
zerothread?id=115587 there is a lot of info on fiberglassing things. Don't take my word for it, because I may be wrong, but I think all you need to do is cut a piece of fleece cloth to go under the speaker and form it however you want it. Then you lay your resin and fiberglass, and keep building it up.
I just fiberglassed a dual 10" box I made, and it really wasn't all that hard. It's hella messy, especially when you sand it, but once you start you will catch on fast. If it's possible do it outside of the car because resin STINKS.
Good idea though, Im looking forward to seeing how it turns out.