If I were going to make something like this for home use (particularly, if I planned to use it several times per year) I would:
Find a local welder
Use heavy, (about 1-inch) square or hexagonal rod stock for the actual spit, and find some square or hex tube-stock that will fit over it, to make the fittings. This way, when you make fittings for the spit, you can "lock" them, so that they will not rotate loose.
Make collars that are round on the outside, that will fit into semi-circular "rests" in the cradle part of your rotiss. Set them up with set-screws (thumbscrews or eyelets) that lock them in place once they're slid onto the spit.
Make a few Cross-Bars that will slide onto the spit (at least three - one for each end, and one that can support a sliding counter-weight, if needed). The end cross-bars can be secured to the pig carcass, in order to keep it from rotating on the spit (sort-of like BIG chicken-tongs on a regular spit, but make sure they're SOLID). A sliding counter-weight should allow you to nearly balance the rotation, and make adjustments. You will probably want one or more relatively heavy weights that can slide towards or away from the spit, at a variety of angles, in order to compensate for the mass of the pig carcass being off-center.
Otherwise, if this is a once or twice per year affair - I saw a link to a site (Cuban Style?) where some guys used metal poles and "butterflied" the pig carcass down the inside of the backbone, and secured it with metal poles and chain-link fencing. Then, they made a fire pit with about three courses of CMU (concrete blocks), set the bound pig across it, and flipped it every half-hour or so.
