I have also been interested in building one. I have most materials, just need the time and motivation. I wanted to build one for mainly baking. I hear you can get 400 degrees pretty easily. Many dishes that are cooked in them have to have some ventilation because of the high heat they get.
There is a kitchen store in town that has one set out cooking things during the summer months. I have seen them doing large squash in it before. Seems like a cool idea.
You not going to get 400 degrees easily.
I researched and built one out of two boxes, rock wool insulation, glass, mirrors, etc.
Full sun would get me 285 to 300 degrees. I cooked all sorts of things in it, including a huge pork butt which came out great. Always use a meat thermometer when cooking raw meat.
I have cooked lasagna, cookies, soup, chicken, eggs, ribs, etc., with it and had a ton of fun.