Here's the cowling after the big cut removing the stock air intake area. You can see my temporary stabilizing strips.

I started gluing 1/2" sheets of foam in the approximate shape of my desired end result. I ordered the foam from Aircraft Spruce and glued it together using 3M Super 77 spray adhesive. I found that I needed to use multiple layers of thin foam as opposed to the thicker stuff because the thin stuff makes the bend much easier. I ended up changing my mind on the shape at the spinner as you'll see in pictures to follow.

This is the basic shape I decided on for the spinner end of the lower cowl. My goal is a smiley-type intake similar to a real P-51. I ended up using clay to approximate the final shape.

As a preview of things to come, you can see the first wave of Bondo creeping down the hill.

For comparison purposes, here's the original intake scoop resting on my newly designed intake.

Here's what it looks like after a few more layers of Bondo and a little shaping with a rasp and some 80-grit sandpaper.

For those of you grasping for your chest at the thought of using BONDO on an airplane, fear not. The Bondo is ONLY for fiberglass layup/molding purposes. Once I achieve the final shape using the foam and Bondo method, I'll lay-up multiple layers of 6oz. fiberglass cloth, let it cure and then pop it free from the mold. I'll then tear out all the Bondo and foam and then glass the new part directly to the existing cowling.

Keep reading the updates to see how the process works. I'd never want to leave this much additional weight on my airplane! This much Bondo is HEAVY!

You can see the basic shape I'm going for in this picture. Obviously the part up near the spinner still requires some serious attention. Getting to this point required some serious sanding and about 4 different applications of Bondo. The first couple of applications were pretty much slobbering the stuff on. After that, It became a more artful process of filling the smaller gaps and fine-tuning the shape.