From my Girl Scout days.
BOX OVEN
Choose a sturdy box hat has the lid still attached. A beer case box is an excellent choice.
Place the box on one side.
Line the entire inside with heavy duty aluminum foil. Make the foil double on the side that will be the bottom.
Place four empty soup can inside to support the cooking pan. OR Use straightened metal coat hangers to create a cooking rack in the middle of the box.
Distribute white hot charcoal, one for every forty degrees of heat needed.
The charcoals can either be in an established fire pit or can be placed inside around the bottom of the oven.
I always did the fire pit.
Place pan of food on cans or rack.
Close box carefully.
Cooking time is about the same as a home oven.
Having enough fuel to keep warm and cook food in an emergency is an often overlooked area of preparedness. One way to preserve fuel is to use an insulated cooker like the Wonderbox. The Wonderbox was developed in the 1960's by Anna Pearse, a South African philanthropist, for use in third world countries. It has been used for several years through her charity Compassion. In 1977, Women for Peace became the umbrella for the establishment of the Wonderbox project. So there's your history lesson for today.
In the depression when times called for conserving and stretching a budget, people came up with ways to save on cooking fuel costs. By using an insulated hay box they were able to cut down on the cost of cooking and baking foods. Food was brought up to a boil or to a certain temperature and then placed into the hay box. The hay box was usually a wooden box and had lots of hay stuffed on all sides of the enclosed pan that maintained the temperature. This process of cooking worked to allow the food to retain the heat by using insulation.
You can save a lot of your fuel needs on those less than sunny days by using a rocket stove. (see previous posts) But you can save even more fuel by using a thermal cooker (pictured at bottom) or a wonder box. (pictured at top and middle) These are heat retention cookers. 
We have been counseled to store a year's worth of food and fuel where possible. The food I have but fuel is a whole other issue. I quickly realized that it was logistically impossible to store a year's worth of any kind of fuel for a family as large as mine. Especially considering the types of food we store, long grain rice, dry beans, etc. Items that have to boil for hours to be edible. I was looking at mountains of propane, or charcoal, or whatever. Just not feasible.