The poly batting melts, or will if you continue to use the same area all the time. Cotton batting is the best to use.
I have two layers of an old cotton flannel blanket on mine. That way it is all smooth. I do buy the silver looking cover. I know Kmarts carry it. Probably the others too. My iron had fallen off and made a burn mark in my other carpet but that carpet is no longer here. When Mother passed, I did manage to get her old wooden one and it is so sturdy compared to mine. Of course, I was married in 1962 and I still have the old metal one. I think I have my iron and board in the attic from when I was a little one. haha Memories are good.