Do a Google for hog trap plans and you'll get all kinds of info, drawings, different designs, etc.

we made it 50 feet of fencing with a 16 foot by 5 ft trail to set off the trap with saloon doors..came out great now just have to catch some pigs

The saloon doors work great we just use heavy bungee cords to keep them closed. We prop the door open for a couple weeks to get them used to going in and out and then set it. We've caught over a dozen at a time with a trap like that.