Cast iron cooks well and retains heat. With the glass top, you actually have an oven inside of an oven and you can see your food cooking. Cooking bags can be used for those bigger foods, such as turkeys, roasts, etc. They are very durable and can be purchased at most markets. Be sure not to tie these real tight as they expand when the heat can't escape at all.
Good reflectors are very important.
Make your reflector surface as large as the area you are reflecting into. Reflectors can be made out of aluminum foil, reflective mylar, glass mirror, polished aluminum, stainless steel or any item that reflects light well. We use mirrors because you can clean glass easily and repeatedly. This is a strong point, although they are cumbersome for portable ovens. Try to use at least double strength glass.