I have done this a couple of times. The photo below is for when I did a half pig and you can just about see the arrangement

A halved oil drum (angler grinder and caution), joined together, forms the fire pit, although this could just be a dug pit.
Two sturdy wooden fenceposts support a steel crossmember - I used a piece of round hollow steel bar about 1" diameter but if I were doing it again I would go bigger.
The bit we were quite proud of is the mechanism to "clamp" the beast onto the rod. It is made of square section steel a bit larger than the round rod and can be locked in place with clamps. It supports thinner metal bars which go through the pig either side of the central rod and stop it just rolling over to wherever it wants to be. It can be rotated in 4 positions at 90 degree intervals.
Thet trick to cooking it is to do it over a very long, slow fire, say 8 hrs or so. Score the skin carefully and thoroughly with a stanley knife to get good cracking - do every bit of skin you can see! Salt it well half an hour before you start cooking, then wipe it down and salt again. This draws moisture out of the skin to get it really dry. If the crackling starts to brown up too early, raise the pig or cover with foil. You can always lower it for half an hour at the end to bring up the crackling, but once its bunt, its no good to anyone.
Let it rest for half an hour if you have time. Carve it on a very large table. Pull all the crackling off and get one of your helpers to chop it with a cleaver or sharp knife - this in itself is a job and a half. Then the meat will practically fall off the bones, and the belly and loins will carve beautifully. I make an enormous pan of apple sauce and have plenty of floury baps and english mustard on the table. Enjoy!