Pig roasting boxes are a tremendous convenience for enthusiasts who would love to barbecue a whole pig, but aren't keen to dig a big pit in their back yard. Commonly known as a "Cajun microwave" in Louisiana or a "caja China" in Florida, a roasting box is essentially a very large Dutch oven. The outer box is constructed of lumber, then lined with aluminum or steel. A recessed lid holds the charcoal, which cooks the pig from above. Deluxe models add a grill over the coals, for the preparation of other dishes while the pig cooks.
Construct the outer box, using three pieces of plywood measuring 4 feet long and 2 feet wide, plus one more for later when you make the lid. You'll also need two more pieces for the ends that are 2 feet square. Commercial boxes are made of cypress, but plywood is acceptable and much less costly. Screw the box together with long wood screws.
Decide whether your box will have four legs, or two wheels and two legs like a garden cart. Screw on lengths of 2-foot by 4-foot or 4-foot by 4-foot lumber for legs, as appropriate. If your box will have wheels, make the rear legs half as long and drill a hole in them for an axle. Fit the axle with surplus bicycle tires, and screw on two lengths of 2-foot by 4-foot lumber as handles.
Construct a lid that fits loosely on top of the box, with an overhang to hold it in place. It should be 6 inches or 8 inches deep, to hold the coals. Cut 2 inches from the side and end of your remaining large sheet of plywood for the main portion of the lid, and build the sides from matching lengths of plywood 6 inches or 8 inches wide. Screw these together with wood screws.
Make the overhang for the lid by measuring 4-inch wide strips of plywood, and sawing them to a 45 degree angle on a mitre saw. Screw them to the upper edges of the box's lid, and reinforce them with two or three small right-angle shelf brackets on each side. Carve four sturdy handles from scrap 2-foot by 4-foot lumber, and attach them to the lid.
Insulate the inside of the box and lid with your choice of insulation from the hardware or building-supplies store. You need one of the high-heat varieties used inside oven walls or in fireplace installations. You may be able to scavenge what you need from defunct stoves at a local repair shop. If it is in sheets, you can secure it with screws, but if it is in the form of loose fibers you'll need to staple it with a staple gun.
Take the box to a local metalworker or shade-tree welder, and have them install an aluminum or steel liner in the box, and on both sides of the lid. Do not used galvanized metal, which creates toxic smoke.
Drill a small drain hole at the bottom of the metal-lined enclosure for rendered fat to escape. Most users prefer to drill a second hole for a barbecue thermometer. Seal around the thermometer with stove cement.
Cover the bottom with one or more racks, held off the bottom by bricks or some similar arrangement. To use the box, place a whole suckling pig, a lamb, or several smaller food items on the racks. Build a good layer of coals in the lid with two or three layers of charcoal, set it in place, and roast the pig according to your favorite recipe.