The trouble with any kind of catch that requires you to overcome resistance to shut the door, rather than take a latch of some kind off, is the wind will always overcome it on a blustery day. I've got a magnetic catch on my garden gate at home - it's fine on most days, but on windy ones there's enough force to overcome it and slam the gate shut.
Hence I'd suggest either the one I posted up or the type Andy'n'Kim / Steve & Denise posted - one where you have to physically release the catch.
As for bits of string - they would drive me mad!
