Hold on, I've been using corks for ages without any mold problems. I just give them a quick soak in diluted bleach, let them dry, and they're good to go. They don't compress if you use enough of them, and the airflow is fine because they're round. Those plastic risers always end up warping in the heat for me, and then the pots wobble. I guess if you maintain the corks properly, they last just as long. Plus, it feels better to reuse something that would otherwise be trash.