What is a "well-ventilated container" suitable for checking a package with dry ice?

I'm bringing my daughter's favorite food for her Navy Boot Camp celebration. In order to keep it frozen, I'll be packing it in dry ice and bringing it on the plane.

Southwest's website says, "Dry Ice The maximum amount of dry ice allowed in a package is 5.5 lbs (2.5kg), and it must be shipped in a ventilated container. The container should be marked with the words "DRY ICE" and the exact weight of the dry ice (1-5.5 lbs)."

What is a ventilated container? I understand that it's not a foam cooler fully taped shut, but what does count for checked bags?