A cardboard box oven works by trapping the heat which rises from hot charcoal briquettes. It is very useful for car camping, but is not suitable for backpacking. I learned about cardboard box ovens about 20 years ago from Julie Fukuda, a legendary Scout leader who has been a long term inspiration to me and to many others.
Making the Oven
Pick a sturdy box of the right size. It has to be long enough and wide enough to be at least two inches bigger than your baking pan so that there is room for the hot gasses to rise and surround the baking pan. (A standard jelly roll pan is 13” x 9”.) It also needs to be high enough to accommodate the pan and be elevated above the hot coals. On the other hand, it should not be too big since that would make it harder to heat and transport. The photographed oven was made out of an 18” x 18” x 18” heavy duty box which had the height cut down to about 11 inches. An earlier box which I used for many years was 20” x 14” x 12”.
It is good if the box has top flaps, since these can be folded down to provide additional strength and insulation. Using duct tape, tape down all of the outside seams and the flaps to strengthen the box. Line the inside of the box with heavy duty aluminum foil with the shiny side out to reflect the heat on to the baking pan. Attach the aluminum foil by cutting pieces equal in size to each of the interior surfaces and gluing them to the box by using a brush and a 50/50 mixture of Elmer’s glue and water. About 2 Tbs. of Elmer’s glue should be sufficient.
Using the Oven
In addition to the cardboard box oven, you will need charcoal briquettes, a baking sheet and/or aluminum foil on which to put the lit coals, and something to elevate the cooking pan above the coals. In the picture I am using 4 empty 15 oz. steel cans (formerly used for canned tomatoes) and a metal rack. Lastly, you will need a stick or a stone to lift a corner of the oven off the ground to provide oxygen for the coals.
Start approximately 12 charcoal briquettes. When well lit, scatter them evenly over the baking sheet (or aluminum foil) and around the supports for the baking dish. Put the baking dish on top of the supports and invert the cardboard box open side down with one edge propped up slightly by a stick or stone. Since the hot air rises, this gap at the bottom does not really lose heat. With 12 coals the temperature in the box should be approximately 350 degrees. An oven thermometer with a wire can be useful in monitoring conditions inside the oven.
Check on the progress of the baking by lifting off the cardboard box. Do not do this too often since every time you raise the box, you lose all the trapped heat. If you are baking something which takes longer than about 30 minutes, you may need to start some additional briquettes so that there is a continued source of heat.
Almost anything can be baked in a cardboard box oven. The one restraint is that paper burns at 451 degrees, and therefore you do not want to get the oven too hot. I have never had any problems with burning of scorching of the oven, but it is possible if you use too many briquettes or put them right next to the cardboard.