Its pretty much doable with a pvc pipe and some mesh, slanted on the side so rainwater doesn't get in. Its to let out all the gases that ripen the vegetables.
Heres a very interesting DIY root cellar that someone posted on their blog. Very cool, gives you an idea of how to create one. Wish he had been more specific with step by step instructions, as it kind of jumps from one stage to the next. Also, factors he didn't talk about were placement, wood vs...