Anything that creates a more or less impenetrable barrier is fine. Traditionally it was probably large, wet leaves, or something like wet corn husks. I’ve seen people use parchment paper or just a pot lid that’s smaller than the pot and fits in snugly directly atop the rice. I understand your objection on the plastic, but it likely doesn’t get hot enough to melt or vaporize anything. I think most people these days use it because they have so many “leftover” plastic bags around the house.