The easiest one I know of requires 8 x7' Tposts and some hog mesh, drive the T posts into the ground to form a circle secure the mesh to the T posts then cut a hole in the mesh, on the inside of the trap cut a larger piece of mesh and secure it at the top of the hole, place bait into trap.
The pigs will work their way around the trap till they get to the flap as each pig enters the flap will drop behind them trapping them in
Of course you can build the trap as big as you want it by using more T posts and mesh.
I used this kind of trap back home in Aus very sucessfully with sometimes having 20 or 30 pigs in the traps. We had a trailer with a coverd ramp backed that up to the opening then pushed the pigs into the trailer, then it was off to the slaughter house with them.