The flat spot on the overall shape provided a great spot to put the first spars on the outside of the structure. The hole for the vent is an 11" inch vent fan. To provide for enough area to screw in the vent uint, I doubled the wood around the hole. The screws in the outside edges get removed once the glue dries. For this project I made the spars 1" thick to match the thickness of the XPS foam that will be used to insulate the camper.

Here you see the spacing of the cross spars. The outside pieces will be shaped using the templates that you made at the beginning of the project. This provides you with an inside curve. I then cut the outside curve about a 1/2 inch too thick. Once the glue is dried on the outside side panels, a flush trim router bit can be used to cut the final shape.

In this picture you can see the completed structure just prior to adding the side panels. All of the bracing in the middle is to support AC ducts, tail lights, and license plate lights/mount.