Edit: here's the model I'm taking dimensions from, for comparison. This is the 6" transom variant.
I think your render is very attractive, but what will the motoring public think?
I thought of a construction technique that might allow you to lay up a composite monocoque tail extension without any downtime for the car.
Build a wooden frame in the shape of the tail extension, on a wooden stand that puts it at the right height. Attach styrofoam boards to the frame and sand them to shape. Park the car in front of it whenever you want to work on it. You'd probably want to leave the rear bumper cover off, and take the taillights off when you need to. Would it work?