i know that an outdoor breeding project seems like the most natural thing to do,and in essence it is,but here are a couple things to consider:
no matter how secure you think you can make it,snakes WILL escape.if you're doing this with the idea of containment in mind, but want to give the animals lots of space, i'd build them a larger cage.
outdoors they will certainly be affected by parasites and pathogens-that is what happens in nature.there is no way around this.
predators will enter your breeding area and eat your snakes.
in a captive environment we have the luxury of climate control-in nature we have no such luxury(think of those nice florida hurricanes!)
if by chance the snakes weren't indigenous to the environment(and yes,i know corn snakes are)they sould escape and introduce forign pathogens and parasites to which your local ecosystem might have serious trouble with-they could also mess up the bloodlines of the native fauna by uncontrolled hybridazation.
there are almost certainly many other reasons this wouldn't be the best idea,though i feel that your heart is in the right place.best of luck!