do you want to do it as a whole pig or quartered? makes a big difference in the size of hole to dig.
I've seen it done but never tried it myself. basically dig a hole and build a fire in the bottom, a big fire, let it burn down to where it is full of hot coals.
They quartered up about a 120 lb hog, placed the quarters & trimmings into brown paper grocery type bags and crinkled it up tightly around the meat, then put that bag into another brown paper grocery bag with the inner bag opening towards the bottom of the outer bag. they then wrapped those paper bags into burlap bags and tied wired around the outside to hold it all in place. Right before putting into the pit, soak the burlap by submerging totally in water for 30 seconds or so.
key point is to get the meat on the coals and get it covered, air tight as possible as quickly as you can after putting the burlap wrapped meat on the coals!!! Otherwise the burlap will start to burn, then the paper bags, then the meat ... and you find out hours later you have a bunch of burned meat!
they used corrugated sheet iron, then covered that up with the dirt they had dug out of the hole. they let it cook for about 16 hours before shoveling off the dirt and pulling the burlap bags out of the pit and placed in an ice chest to transport to cutting blocks. They also tossed in a couple of briskets too that came out extremely moist and tender. The pork basically fell off the bone upon removal from the bags.