When you lay it flat tie a small sash cord type rope to the top of the door that runs across the width of the trap (short side) and out one of the squares. Make the rope a hair over double the width of the trap. Basically when the door is shut, your rope and snap should still be outside the trap by a foot or so. Tie a brass snap on the end. Use that rope to pull the door open, snap it open, and snap it closed if need be. Just pull the rope toward you and run it to the back of the trap until the door is wide open. Snap it to the nearest part on the panel.
Take some of that dark green or black string like you use for trot lines and tie it to the bottom corner of the the trap. Run the string outside the trap, just like you did with your rope but at the bottom, and along the side of the trap to the back corner. About 2' or so from the corner go back thru the trap and aim for about 2" along the back wall of the trap. Take some stiff slick wire and make a loop on one end for you string to tie to. On the other end bend a 90 deg hook in it. Hook that hook in to the back wall as close to the bottom as possible. You adjust how hair trigger the tripe wire is by how much bend you put in the hook.
Once you get it hooked on the back wall. Go back to your rope and unsnap it. Slowly give the rope slack to be sure your hook is holding the door open.
If you catch pigs the first thing you do when you walk up is secure that door. You jump out and walk right to it and tie it shut with rope, snaps, zip ties, or some thing. Every second you waste is a chance they can hit that door and potentially pop it open.
As always, corn for as long as you can stand it before you actually set the trip wire. Never set the trip wire 2 nights in a row. You will also want to cut that floor out or cover it with dirt to increase your odds.
Good luck