I've used old nursery pots stacked on top of each other, in progressively smaller sizes upwards, with the center cut out of the bottoms, leaving about an inch of material near the outside wall of the pot. I then use sheet metal screws to secure the pots together, creating a large cone. I then fill with either straw or bark mulch, and place an uncut pot over the cone as a cap. There's enough ventilation to keep air circulating and the "cap pot" keeps any serious moisture out. It works for me here on the west coast where rain is my main concern. Here is a picture of the end product:url=http://www.bananas.org/gallery/showphoto.php?photo=37677&ppuser=5590]
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