Hinge a door from the side and have it where it closes at an angle. Use a big bungee strap to pull the door closed. The bungee will give enough to allow the door open when a hog pushes on it but hold it closed on it's own. When the hogs come in they push on the door and push it open, then when they try to leave they push against the door it will wedge it closed tighter. New hogs can come in and push the door open but the hogs inside the trap cannot push the door open. We've had hogs wedged in the traps where we couldn't get anymore inside. We have other traps that have a sliding door straight down, though they learn to push the door up if you don't have a latch to prevent it from sliding back up. We use a spring loaded pin that slides into the door when it slides down. Until we added the pins we used to listen to the hogs sticking their snouts in the door and raising it up until they could slip out.