For your regular door, use large hinges, and put three on the door, not just two. Sink your screws into the door so it will close flush, bolts on the outside of the hinge. (Don't forget the Loctite) Put a 2x4 on the outside middle of the door from left to right and drive an eye bolt from the inside into the 2x4 by the hinge side of the door. Anchor a trampoline spring to the inside of the door frame and to the eye bolt, you now have a spring door that will stay tightly closed.
When you get out, use two hasps on the outside instead of just one. In the off season, remove the spring and screw it shut with several screws on all sides, this will keep it from warping.
The spring also allows me to prop the door open to let a breeze through without it flapping in the wind. I will try to remember to take pics next time I swing by the lease.