Being a fan of sounding rockets, I did a 4" a few years back. After getting the scale documentation from NAR, along with the drawings from "Rockets of the World", and taking measurements from the Estes model, I was ready to start. But how to do the fluting/fin shroud?
I use basswood quite a bit in rocket construction, stronger than balsa, with not to much additional weight. Best of all, with its tight straight grain, much easier to finish. So basswood it was. I started by cutting 1/4 inch stock in 16 pieces, about 1.25 by 12 inches long. I then put a 11.25 degree bevel on both edges. The curvature was sanded into each piece by wrapping a piece of sandpaper around a section of 4" airframe. The edges were than trimmed to size and glued up. It was glued up with CA, on a piece of airframe tubing, covered in wax paper, so I could just slip it off when done. As the flues are not the same width, I painted the outside, to help see the progress in sanding each flue to the proper size. The bevels were put on the end, fin slots cut and it was done. While it sounds simple, it was a lot of work, but in the end it worked out well.
The trim pieces that go around the fins where they meet the shroud were cut from styrene strips and were attached after the rocket was painted. As a bit of overkill, to get the look of the screws, real screws were used. "Acme", rail guides were used, as they are close to the guides on the full size rocket.
Mike