I do NOT want to railroad your idea into a ditch but since you are new here, I will give you a few pointers. First off, a lot of people on here do the old "Yeah, I'll buy one" and when the part is made and all the labor is done, they run for the hills......take it from me and some of the other vendors on here who have been screwed over. Build a kick-ass setup for yourself. Don't worry about production until your car is perfect. THEN if people want it, you can start talking price. Most people on here will want to pay very little other than being free.
As far as a diffuser goes, Honda S2000 diffusers range from $185 for aluminum to $800 for carbon fiber. They fit the Fiero perfectly under the trunk and have removable vanes for customizing. I don't know if you would want to buy one for yourself and tweak it with additional carbon fiber or fiberglass for the look/function that you want. You could be saving yourself a lot of headache by just buying one pre-manufactured.
Chris (Doublec4) on here installed this one on his car. You may want to do something similar and then work off of it:
These are $250 plus shipping on Ebay right now. They will require tweaking but again, it may save you some time from starting from scratch:
[This message has been edited by IMSA GT (edited 07-14-2013).]