An oven box is a cardboard box lined with tinfoil. You can cook anything in here that you would normally cook in your kitchen. To make a side unit like the one pictured above, you will also need a lid lined with tinfoil. The lid will act as the door and the size of the box will determine the size of the meal that you'll be able to cook. When you line the inside of the box, leave enough tinfoil sticking out to duct tape to the outside of the box. You do not want tape or glue on the inside of your box, as the fumes from these products will get into your food. Also make a couple small holes on the sides for ventilation for your fuel gases to escape.
Once you have your box lined, punch three small holes at each end of the box. You will need three untreated straightened coat hangers that you will thread through the holes in the side, as this will make the rack for your oven. You can also use pop cans as pictured above, using a ready made rack.
For fuel you will use either hot briquettes or hot coals from a camp fire and put them in a pie plate or something similar. If you use the briquettes, one briquette equals to 40 degrees of heat, so if you required the temperature to be 320 degrees, you would need 8 briquettes, and they stay hot for around 40 to 45 minutes long. Coals don't last as long, so you'll have to start checking on them after about 20 to 25 minutes and start replacing them with more hot coals as required.
If you set it up like the picture below, you will not need a lid, you would just place the box over top of your cooking tray and coals. You will have to prop the box up at one end using small rocks, this is to give your fuel some ventilation. Using this method, you would loose most of your heat when you lifted the box up, to check on your food being cooked. Once you got use to cooking with a box like this, you would have a better judgment as to when your cooking time was up, for individual items.
If you want to install a meat thermometer, punch a small hole on one of the sides and about 3/4 of the way up. This will give you a good indication as to what the inside temperature is of your box.