Make sure to build your cinder block oven in an open area cleared of all vegetation with a nice flat dirt surface as a base.
Build the perimeter of the oven with the cinder blocks about 4 cinder blocks lengthwise and 2 cinder blocks widthwise, and 4-5 cinderblock high, adjusting as necessary to accommodate the size of the butterflied pig you are roasting. On either short end of base, position one of the cinder blocks so that the open cavities are visible to allow airflow through the oven from one end to the other. (See picture).
Stack a column of cinder blocks inside each interior corner about halfway up the height of the oven to act as support posts for the metal rack that will hold the whole hog to roast. Create 2 more columns halfway down each long side if necessary as well for added support
Light an extra large chimney of charcoal.
Spread a ring of charcoal around the inside perimeter of the cinder block oven, about 3-4 briquettes high and wide, but NOT directly below where the pig will be positioned.
When the charcoal in the chimney is lit, pour about half the lit briquets on top of the ring at one short end and half on top of the ring at the other short end. This will create e low and slow burn towards the center of each long side.
Wearing protective gloves, place the aluminum wire rack and pig, skin side DOWN, on the support columns in the cinder block oven.
Make sure your temperature probe sare in place in the shoulder and ham and place another temperature probe near the pig to monitor the cinder block oven chamber cooking temperature.
Place the large aluminum sheet metal on top of the cinder block oven with a brick or two on top to keep it from blowing off.
Position your smaller sheet metal or plywood prices near the open cinder block cavities to positioned at either short end of the base. Start with them covering the openings about halfway and adjust as necessary to maintain a cooking chamber temperature between about 225°F and 275°F.
You will need to remove the lid and add additional charcoal to the corners and inside perimeter every 1-2 hours as necessary. Depending on the size if your rack you may need to lift it up temporarily with the hep of some friends wearing protective gloves, or sprinkle the charcoal in around the edges between the rack and the cinder blocks if there's room.
Continue roasting the pig until the internal temperature probes in the ham and shoulders reads 180°F.
At this point, open the cinder block oven and flip the pig over carefully with the help of some friends. Score the pigs skin with a sharp knife and continue cooking until the internal temperature of the hams and shoulders reaches 187°F.