Patrick,
I use a styrene product from "Evergreen", they manufacture a line of styrene products ranging from sheets to rods and strips. It started, I believe, as a product for Model Railroad enthusiasts however I see it referenced here as well. It, or a similar line, should be available at most Hobby Stores. In lieu you could use scrap from your spares box or even sprue. All you need do is glue on your extended profile, fill and sand to the new profile you want, and you should be in business. Assuming you can remove the cowling it's a lot easier then filling the present gap and maintains the separate "panels" on your model. I'll dig around today and see if I have a photo or two that might help.
Cheers,
Lance