This is one way to make a great box oven. I used my oven for about 10 years before it needed to be replaced. You can make all sorts of food in this oven. My favorite is Chocolate Chip Cookies, which need to be cooked in a circle so the middle cookies don't get burned while the outside ones aren't cooked at all.
Step 1: Get Yourself a Nested Apple Box and Some Other Supplies
Gather your supplies. You need kitchen scissors, a box knife, 50 feet of aluminum high heat tape, and about 30 linear feet x 18" wide heavy duty aluminum foil. Get one of these nested apple boxes. The brown base should fit inside the top. Separate the box pieces. Using the top section, cut open three sides of the top to create a fold down flap as shown in the picture. Make sure to cut away from the body in case the box knife slips.
Step 2: Fortify the Bottom of the Box!
Using the interior box piece as shown in the first picture, see the area marked with the 'X'. This is the area where all the heat of the oven will be concentrated. Cut 5 - 12" sections of the 18" wide foil. Line up the foil and fold it in half. Use 4 - 6" pieces of aluminum tape to secure the foil to the bottom of the oven.
Step 3: Fortify Any Holes
Put the interior box back inside the outside portion of the box. Use 4"-6" sections of tape to cover both sides of the handle openings and the flap portion of the outer box.
Step 4: Cover the First Section of the Box
For this section you need about 10 linear feet of foil. Use 6" of tape the secure the foil to the back of the box. Smooth the foil over the box securing it with 4" sections of tape as you cover the box. fold over the flap and cover the inside of the box. Keep working until you get back to the side of the box you started on. Use small sections of tape on the inside and make sure to make the corners as square as you can.
Step 5: Add a Second Section of Foil
For this portion you need about 8 linear feet of foil. Start on the short side of the box. Tape foil to the box with a 4" piece of tape. Smooth the foil up the short side, down the inside of the oven, over the bottom, and up the other side. Secure with tape as you go along. Cut the corners as you go around the top and tape around the edges to stop the foil from separating. Tape all the foil edges so you don't have any flaps or openings on the inside of the oven. Smooth the foil until you run out of foil and tape down all the edges. Try to make all corners as smooth and square as possible which will help keep your oven looking nice for a much longer time.
Step 6: The Last Section of Foil
For this section of project you need about 4' of foil. Wrap the foil and secure with tape. When I taped my foil on the top of the oven I centered it on the oven. I secured both sides like a present with the tape. I made sure to cover any holes in the foil.
Step 7: Secure the Flap
The oven sits with the flap be opening to lay flat on the ground. I like minion duct tape, so I cut 3 4" pieces. I put on piece on the flap, one on the top, and one connected the two. This oven needs to have a gap when you put in the charcoal for off gassing.
Step 8: Furnishing Your Oven
For this step you need 4 empty aluminum cans, 4 - 6" x 18" wide pieces of foil, sand or pinto beans, small grill, small aluminum cookie sheet, and two stackable pie pans. First fill the cans 1/2 - 2/3 full of pinto type beans or sand. Then tape the top of the can with a 2" piece of aluminum tape to make sure the beans and such don't fall out. Wrap the cans with foil and secure them with tape. Fold the ends over the cans and set them aside. Put the pie pans into the oven and arrange the cans in a square. Place the grill over the cans. When the coals have been heated, put them in the pie pan. Heated coals are 50 degrees each, so add as many as you need to make the correct cooking temp for whatever you happen to be making. The box oven will get hot on the bottom, so be careful if you move if directly after cooking.