First step is to create an accurate template of the contour. I already trashed the template so can't show it entirely. But the way a template is made is to take a piece of thin plywood that is larger than the complex area you want to fit (in this case the roof line) and cut it closer and closer to fit. If you over cut you can fill the line back in with heavy paper and tape or more wood. Eventually you get a scrap that fits the complex portion of your overall shape.
The rest is straight lines usually and these can be "filled in" with strips of thin material. Typically templaters in residential scale use 1/8 or 1/4" plywood cut in 3" wide strips that are 8 foot long. These can be cut with a razor knife and snapped. Then they are positioned around the perimeter of your future panel and screwed together. The template should exactly fit the interior of the space you are filling.
After that it is transferred to a real piece. Then after test fitting you can use that actual piece (not the template) for cutting.
Photo shows first wall section cut from the template (now referred to as template since it is simply a more solid reproduction of it) that is then going to be duplicated onto the one below it.
First I rough it out with my jigsaw as the template bit on the router will last much longer if it can trim rather than plow through its full width.
This shows a dual function bit which is a trimmer (due to bearing on end) and a template bit (due to bearing at top). One allows template to be on top of your piece and the other requires the template below. Personally I like the template on top as it allows you to see how it is placed.
It took me a little over an hour to produce eight of these for the project. For the most part they fit the bus fairly consistently. I made them tight fit and ultimately want them about 1/2" off the floor so I can trim to fit better if I want.
The material is a lightweight mdf board, Truepan. I like it for walls as it is light but solid. Being solid make it easy to do all future fastening of cabinetry, molding, fixtures and artwork. It is not expensive and I like that I can cut v-grooves or beadboard in it if I want.