Check out some of the threads on fiberglass and such that are here..look at the ones where we used fleece or old t-shirts for the layup..
Be really good to yourself and test the resin to see that it sticks in an inconspicous spot..
Then lay a layer of fleece or t-shirt over the dash and soak it with resin so it is completly saturated..use bondo in the corners tha tyou wish to change and glass over any holes that you will not need..
Once that is all done spray a coat of gel-coat and sand that until you are satisfied withe the finish and then pull your mold from that..
Be careful not to change the size of the piece very much specially in the areas where it has to mount or fit up to the existing car..Check out these guys for materials