I saw something on the internet which may help.There is an outfit which has done a 3D model of this car and they sell some kind of download of the drawing. Maybe this would help
http://gunpoint-3d.com/model-Sdkfz222.html It is a complicated shape ,not easy to do.Another way is simply buy a Tamiya or Dragon Sdkfz 222 model in 1/35, assemble it, then very carefully measure it and then scale this up. Maybe you can do an actual floor, side walls,upper small side walls, then a partial top ,the correct width, then start using plywood to develop the templates for the rest. It`s a tricky shape