After a good deal of cutting and filing I got the basic shape of my cover plate to fit into the underside of the dash. To make it a bit more of a snug almost "Jig-Saw-Like" fit I re-taped it into position with painters tape, masked off any exposed areas and then coated the painters tape in the areas that I plan to put more layers of fibreglass and then coated the area with a layer of white glue to act as a barrier between the tape and fibreglass. Once the glue dries I'll glass over those areas and make kind of a "Lip" so the underside plate will fit nice into that area. My guess is that when I do a test fit I will need to extend the area of my cover plate that fits closer to the stock OEM dash. But I'll cross that path when I get there.
This little project is turning out to be a lot more work than I had bargained for but it should fit nicer though with all of the extra work. That being said I can't say as I would recommend doing it this way unless you have a lot of patience ;)