Styrofoam cones for winter protection

I'm not sure if I read about winter protection in this listserver or the Zingiber listserver.  Somewhere, I heard of some Styrofoam-like material that was cone shaped and was placed over a tender plant to add winter protection.  Reportedly it added a zone or 2 of winter tolerance.

I've looked in the local Home Depot, local Hardware store, a couple nurseries and have even called a plastic pot wholesaler but can't come up with any information.

Does anyone have any experience with these and if so, does anyone know of a supplier so that I can get the local nursery to purchase a mass of them?

Thanks,
           Les
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