This site http://www.bateau2.com/modules.php?name=Forums
has a forum and somewhere I remember reading about someone making one out of some kind of foam board and glassing it with cloth and epoxy. Not too hard to do.
Jim,
make it as light as you can, you don't want to put too much weight on that T-top and start breaking welds! I would probably go with some type of foam for a shape and then several coats of glass mat and epoxy to make the structure.
Here is an interesting article on making custom fiberglass panels but like MakoMike said - you want to stay light. One option is to huny Ebay for one of those Todd boxes. I have one on mine and they are made of a lightweight HDPE and are not as heavy as glass...